What is a bushfire hazard?

A bushfire hazard is created by:

  • fuel, such as leaf litter and vegetation, and may include forested areas, woodlands, shrublands, heathlands or grasslands
  • the potential for topography and weather conditions to contribute to fire behaviour.

The hazard can influence what a fire will do and how it will behave in the landscape. It is a source of potential harm or a situation with a potential to cause loss.

Drawing showing in forested area to show the bushfire hazard

Planning assumes that a bushfire could start in any bushfire hazard. Therefore, State planning policy at clause 13.02-1S of Victorian planning schemes requires that bushfire hazards be identified, considered and assessed in all decision making where there is the potential for bushfire.

This applies to both strategic and statutory planning.

More information on the State planning policy

Bushfire hazard and risk are different. Bushfire risk is the likelihood and consequence of a fire starting, spreading and impacting on people, property and the environment.

Because planning assumes that a bushfire could start in any bushfire hazard, the emphasis in risk assessment is on the consequence and what will happen as bushfire spreads and impacts on people and property.

The Bushfire Prone Area (BPA) and Bushfire Management Overlay (BMO) hazard maps trigger specific building and planning requirements.

Further consideration of the bushfire hazard at the site and landscape scale is required for all strategic and statutory planning proposals.

What is a bushfire hazard assessment?

A bushfire hazard assessment documents the various types of bushfire hazard.

The assessment:

  • provides information on the bushfire hazard (including vegetation and slope).  Depending on the planning proposal a bushfire hazard assessment could be required at different scales from a site:
    • landscape (within approximately 20 to 75km)
    • local (within approximately 1km)
    • neighbourhood (within approximately 400m)
    • site (within 150m)
  • provides contextual information on a site or area.

This information is typically provided in a report, including spatial diagrams, plans and photos.

A bushfire hazard site assessment provides information on the bushfire hazard on and near to a site within 150m. The site assessment informs the appropriate bushfire protection measures such as defendable space, building construction requirements, water supply and access.

All planning permit applications need to provide a bushfire hazard site assessment, except where an application has been made for a single dwelling under a BMO schedule and some  simple non habitable outbuildings.

There is some scope for the council to vary or waive the requirement for a bushfire hazard site assessment under the BMO. However, this usually only applies to very minor applications, such as a small change to an approved plan that does not change any bushfire protection measures.

Refer to the following document to inform planning decision making:

Templates are available

A bushfire hazard landscape assessment provides information to planning and responsible authorities on:

  • the bushfire hazard
  • potential bushfire behaviour
  • the impact of other relevant strategic considerations in the landscape that are likely to affect a given location such as the road network and nearby safer areas.

The assessment is not only an information tool.  It helps inform the appropriate response to a planning proposal. For example, it helps determine the level of bushfire risk, assists with the assessment of siting and design, and raises questions about access and egress options.

For new development in the BMO, where the risk of bushfire is very high, it helps identify if enhanced protection measures will be required.

The purpose of the assessment is not to predict the outcome of a bushfire event. It provides information that builds a better understanding of the bushfire risk at a location. This helps informed decision making.

The information is typically provided in report format including spatial diagrams, plans and photos.

All planning proposals including strategic planning and planning permits, will need to provide a landscape assessment, except:

  • a single dwelling permit application where a Bushfire Management Overlay schedule applies
  • a single dwelling permit application in an urban residential zone
  • where a council waives the requirement under the BMO.

Refer to following document on how to prepare a landscape assessment.:

Bushfire hazard identification can be complex.

It is recommended you use a bushfire planning consultant or a planning consultant with experience in preparing bushfire applications.

An application prepared by someone experienced in the bushfire requirements is likely to speed up the approvals process.

A list of accredited bushfire planning and building practitioners is available on the Fire Protection Association Australia website (note: this is not a comprehensive list).

Factors that influence bushfire behaviour, including intensity and severity in the landscape

There are four major factors that influence bushfire behaviour:

  • Vegetation
  • Weather conditions
  • Wind speed and direction
  • Topography.

The type and extent of vegetation may help indicate where a fire will start and the direction it may approach from. Vegetation acts as the primary source of fuel for a bushfire and can burn differently depending on the amount, type, arrangement and moisture content.

Interactions between the weather and vegetation largely influence the intensity and duration of a bushfire. The most important factors to consider relating to vegetation and fuel are:

  • Fine fuels such as leaf litter readily dry out, ignite and can be carried as embers.
  • Shrubs, vines and other elevated fuels can act as ladder fuels, allowing fire to climb into the canopies of trees, significantly increasing bushfire intensity.
  • Finer fuels such as grasses burn more quickly, and more substantial fuels burn with greater intensity.

North and north-west facing slopes are more likely to dry out quickly in summer. Therefore, they will ignite and burn vegetation more easily than vegetation on a south facing slope.

Breaking up the continuity of the vegetation can limit the spread of fire. How or if this can be achieved will vary depending on the proposal.

Key consideration for planning decision making:

A fire run is an area of continuous vegetation where a fire can ‘run’ (move) through a given landscape. It is expressed as a distance (km or m). They show how large a bushfire may become before it impacts on a location.

Fire runs located north-west and south-west of a proposal will be of interest to decision makers. Dominant winds in Victoria will push any bushfire in these areas towards a proposal.

Fire runs are shown on a bushfire hazard landscape assessment.

Bushfire weather conditions are largely determined by:

  • Temperature
  • Humidity
  • Wind and atmospheric conditions
  • Drought conditions or the amount of rain.

Hot, dry and windy days provide ideal conditions for a bushfire. A string of hot days dries out vegetation, making it easier to burn. In summer, these are common weather conditions that increase the flammability of vegetation. The drier the vegetation, the easier it will burn.

Low humidity and high temperatures, which are fuelled by hot winds, also dry out vegetation, allowing it to readily ignite.

A simple measure of weather conditions is the Forest Fire Danger Index (FFDI) and the Grassland Fire Danger Index (GFDI). These are used to help determine the fire danger rating.

Fire danger rating semi circle diagram going from low to moderate to Code Red

Key consideration for planning decision making:

Planning schemes use weather conditions to inform an understanding of likely bushfire behaviour. Weather conditions are used in several ways.

When determining defendable space, approved measures in Clause 53.02 Bushfire Planning use weather conditions derived from the Australian Standard AS 3959:2018 Construction of buildings in bushfire-prone areas (Standards Australia).

Sometimes, planning decisions need to be based on a worst case or most likely weather scenario. This is especially so when considering the bushfire risk in the broader landscape and applying the clause 13.02-1S policies. For example, when siting buildings in high-risk locations or in places that may be too dangerous to develop.

A planning decision may require locally specific weather conditions to be analysed before approval is granted. This information will be provided in a bushfire hazard landscape assessment and bushfire hazard site assessment.

Wind influences the speed that a fire will spread, the direction in which it travels and the size of the fire front, its intensity and the likelihood of ‘spot fires’ from embers ahead of the main fire.

Wind direction can determine how a bushfire may ‘run’ (move) through the landscape.

Wind can travel in all directions, but there are typical weather patterns that are likely to influence bushfire behaviour, particularly on days where a bushfire is more likely. A change in wind direction is one of the most dangerous influences on fire behaviour.

In Victoria, the most dominant winds typically come from the north and north-west. They are often followed by a south-west wind change. A wind change like this can cause a shift in the direction of the fire. This means what was a slower burning ‘flank’ (side section) of a fire becomes the head (front) where more intense fire behaviour occurs.

During Black Saturday, a south-westerly wind change at the head of the fire impacted access routes. People’s ability to leave was changed.  New fire fronts were created putting more areas at risk.

Afternoon sea breezes often affect coastal areas. They are caused by the difference between temperature on the land and the sea, with the movement of warmer air towards land.

Key consideration for planning decision making:

Wind direction is a key consideration as part of the bushfire hazard landscape assessment and requires a wind direction diagram.

This provides a visual of the dominant winds to be expected across Victoria. In areas affected by coastal winds add a second wind direction diagram.

Fire spread and wind change in a bushfire diagram - two images one of wind blowing from the north onto the narrow side of the fire; and the second showing a wind change to blowing from the south west and the longer eastern flank of the fire becomes a much larger fire front.

Topography and slope influence the speed that a fire will travel, its intensity and the height of flames.

Fire burns faster uphill. As the slope increases so does the speed of the fire and its intensity. When burning up a slope, flames and radiant heat preheat the vegetation ahead of the fire, drying it out and making it easier to burn.

As a rule, for every 10˚ slope the fire will double its speed. For example, if a fire is travelling at 5 km per hour along flat ground and hits a 10˚ slope, it will double in speed and travel up the hill at 10 km per hour.

Fires tend to move more slowly as the slope decreases. The flames reach less fuel, and less radiant heat preheats the vegetation in front of the fire. For every 10˚ of downhill slope the fire will typically halve its speed.

Diagram showing the fire increases uphill - from 5km an hour on the flat to 10km an hour on a 10 degree upslope

In areas of rugged terrain where topography includes steep slopes and long areas of continuous vegetation, more extreme fire behaviour is likely as it is driven by the landscape. Other topographical features such as gullies in more mountainous terrain can channel fire and increase its intensity and speed.

Other features such as rivers or other natural water bodies may affect how a fire traverses across a landscape. This may depend on their size and aspect, or the direction a fire approaches from.

Key consideration for planning decision making:

Topography is detailed in both the bushfire hazard landscape assessment and bushfire hazard site assessment.

Where the topography is in large hazard areas, the bushfire hazard landscape assessment will need show where extreme bushfire behaviour may arise.

In the bushfire hazard site assessment, topography is used to assess the slope under vegetation within 150m of development. This informs the separation distance required between the bushfire hazard and new development.

Extreme fire behaviour because a convection column has formed

Convection is the transfer of heat through the movement of heated air.

Most of the heat transferred from a bushfire is from convection currents of hot air. This process forms a convection column of rising hot air and a smoke plume above the fire.

The convection column can carry ash, embers and pieces of burning fuel, as well as preheating the vegetation above the fire (higher shrub layers and tree canopies). Large convection columns can produce severe weather events including cyclonic wind and lightning.

Image of wind flow, fuel and spiralling convection column

Key consideration for planning decision making:

The conditions in the broader landscape around a proposal influences the size to which a convection column can grow and in essence the destructive potential of a fire.  The conditions include the topography and the extent and ability of fire to run through vegetation.

Therefore, the planning system requires the bushfire hazard to be assessed at many scales. This allows for planning proposals to fully understand and respond to the bushfire risk.

How bushfire impacts a proposal

Understanding the impacts of bushfire, and mitigating them through planning decision making is important to manage the risk.

Likely bushfire behaviour described as part of a bushfire hazard landscape assessment and bushfire hazard site assessment helps determine the way a bushfire may impact on a proposal.

There are five main forms of bushfire attack that are addressed in planning decision making.  Each of these are described below.

Forms of bushfire attack relevant to the bushfire hazard assessment:

Extreme fire behaviour

Characteristics of extreme fire behaviour

  • Fires will likely form large convection columns with winds strong enough to damage buildings and blow over trees restricting access.
  • Occurs from fire behaviour in the surrounding landscape, including where it interacts with the hazard in close proximity to a building.
  • Arises in high risk landscapes, with long fire runs, steep topography and vegetation in a mostly natural state.
  • Influenced by fuel loads and drought conditions.
  • Extreme ember attack will occur.
  • Associated with weather as seen on Black Saturday.
  • Any fire that starts and takes hold will be so intense that life safety may be seriously compromised.

Image of house surrounded by high flames on three sideses

Key consideration for planning decision making:

Places subject to extreme fire behaviour require careful consideration and decision making. The state’s bushfire planning policy at clause 13.02-1S and the need to prioritise life will mean many planning applications in these areas may not be appropriate. If a proposal does proceed strengthened resilience is often required.

Localised flame contact

Characteristics of a localised flame contact:

  • Occurs from the hazard in close proximity to a building (up to 50 metres).
  • Direct flame contact from individual elements, such as vegetation, fences or structures.
  • Burning elements on neighbouring land may expose a proposed building to localised flame contact.
  • Can occur in areas where the vegetation is modified or is managed as a garden.
  • Not direct flame contact from a moving fire front.

    Image of house with low flame in garden next to house and flying embers overhead

Key consideration for planning decision making:

Local burning elements can be harmful to people and structures, by exposing them to flames. For example, when people are actively defending a property, or if residents are sheltering in the open due to building failure.

Providing bushfire vegetation management standards minimises elements that can catch fire in the local environment.

Flame contact from fire front

Characteristics of flame contact from fire front:

  • Occurs from the hazard in close proximity to a building (150 metres).
  • Direct flame contact from a fire front where vegetation is in a mostly natural state (such as in national parks).
  • Occurs when a building is in close proximity to the vegetation.
  • May arise in lower risk areas (such as from a local park) or in higher risk areas (larger vegetated areas such as forests and coastal reserves).

Image of house with direct high flame contact on side and at rear

Key consideration for planning decision making:

Bushfire planning provisions provide for the separation of a proposal from bushfire hazards to avoid flame contact from a fire front. Any proposal that would result in flame contact from a fire front will need to be carefully considered. Flame contact from a fire front is an indication that a location may be too dangerous to develop and that a proposal should not proceed.

Radiant heat

Characteristics of radiant heat:

  • Occurs from the hazard in close proximity to a building (up to 150 metres).
  • The heat you can feel from a fire.
  • Can ignite surfaces without flame contact or ember attack.
  • Dries out vegetation ahead of a bushfire so that it burns more readily.

Image of house with tree in front and flames approaching

Key consideration for planning decision making:

Bushfire planning provisions provide for the separation of a proposal from bushfire hazards to minimise radiant heat on buildings. The level of radiant heat permitted will depend on the use proposed and what planning scheme provisions the decision relates to.

Higher levels of radiant heat need to be avoided. Excessive levels of radiant heat are an indication a location may be too dangerous to develop and that a proposal should not proceed.

Ember attack

Characteristics of an ember attack:

  • May occur from the hazard in very close proximity to a building (nearby trees, neighbouring houses).
  • May occur from fire behaviour in the surrounding landscape.
  • Most common way houses catch fire during a bushfire.
  • Occurs when small burning twigs, leaves and bark are carried by wind, landing in and around a building.
  • Can happen before, during and after a bushfire.

Image of tree and grass at base on fire and embers - leaves, sticks and bark - flying to wards house

Key consideration for planning decision making

Development in the Bushfire Management Overlay and bushfire prone area are required to be constructed to a minimum construction standard of Bushfire Attack Level (BAL) 12.5.  The BAL provides for a basic level of ember protection.

Where ember attack is at higher levels, a greater construction standard may be necessary. This may apply to areas subject to extreme fire behaviour.

Page last updated: 25/01/22