What Is Bushfire Behaviour — and Why Does It Matter for Planning?

Bushfire behaviour refers to how a bushfire moves and reacts to its surroundings — including the type of vegetation present, the local weather conditions, and the slope of the land. Understanding these patterns is essential for predicting how a fire might behave in a specific location, and for determining how best to plan, build, and protect in bushfire-prone areas.

For anyone involved in building or development in regional or semi-rural Western Australia, this knowledge directly influences bushfire risk assessments, BAL ratings, and the bushfire planning conditions imposed by local councils.

Why Understanding Bushfire Behaviour Matters

Knowing how a fire might behave allows planners and bushfire consultants to:

• Predict the intensity and speed of a potential fire

• Estimate the direction it’s likely to move under certain conditions

• Identify higher-risk areas where development may need special design or mitigation

• Recommend appropriate construction standards and asset protection measures

These insights don’t just shape theoretical assessments — they can influence where a building sits on a block, what materials are used, and how access or water supply is designed. For example, a house placed at the top of a steep slope facing dense vegetation will face much higher fire intensity than one located on level ground with cleared surroundings.

The Three Key Factors That Drive Bushfire Behaviour

Bushfire behaviour is influenced by a combination of fuel, weather, and topography. These elements interact with each other — which is why fire behaviour can change rapidly and unpredictably under certain conditions.

1. Fuel

In fire science, “fuel” refers to anything flammable — not petrol or gas, but vegetation like grasses, shrubs, trees, and fallen leaf litter. Several fuel-related characteristics affect fire behaviour:

• Type of fuel: Grass burns differently than dense scrub or eucalyptus forest. Some plants, like dry heath or oily eucalypts, produce more intense flames.

• Fuel load: The more vegetation present, the more energy a fire has to burn — which increases its intensity.

• Moisture content: Wet vegetation is harder to ignite. Dry fuels burn faster and hotter.

• Arrangement: Closely spaced shrubs with a continuous canopy (vertical and horizontal connectivity) help fire spread faster through both ground-level and treetop fuels.

Understanding the structure and distribution of fuel near a building site is crucial in classifying vegetation and applying AS 3959 bushfire construction standards.

2. Weather

Weather can either fan the flames or help suppress them. The most important weather factors are:

• Wind speed and direction: Wind drives the speed and direction of a fire front. It also carries embers ahead of the flames, igniting spot fires far from the original burn area.

• Temperature: High temperatures dry out fuels and increase fire intensity.

• Relative humidity: Low humidity draws moisture out of vegetation, making it easier to ignite.

These conditions can combine to create catastrophic fire danger days. That’s why every BAL assessment uses a region-specific Fire Danger Index (FDI) — in WA, this is set at 80 for the purposes of standardised assessment.

3. Topography (Land Slope)

Fires behave very differently depending on whether they’re burning uphill, downhill, or across level ground. Slope affects both the speed and severity of a fire.

• Uphill: Fires move much faster and burn more intensely as heat pre-warms the unburnt vegetation above. A commonly cited rule is that fire speed doubles with every 10° increase in slope.

• Downhill: Fires tend to move more slowly, though wind can still push embers downslope.

• Flat ground: Fires move steadily, driven mainly by wind and fuel conditions.

When assessing a site, we measure the slope beneath the classified vegetation, as this directly influences the potential fire behaviour directed toward the proposed development.

How Is Bushfire Behaviour Used in Risk Assessment?

These three factors — fuel, weather, and slope — are combined during a Bushfire Attack Level (BAL) assessment, which forms the basis of your site’s bushfire construction requirements.

We use this data to:

• Classify vegetation on and around the site

• Measure setbacks from dangerous fuels

• Assess slope and aspect to understand how fire may approach

• Determine the BAL rating, which ranges from BAL–Low to BAL–FZ (Flame Zone)

Each BAL level aligns with construction upgrades designed to reduce the risk of building ignition — from ember protection (BAL–12.5) to flame contact resistance (BAL–FZ).

To learn more about the process, check out our related article:
👉 Understanding BAL Ratings: What It Means, How It's Calculated, and What to Do Next

In Summary

Understanding bushfire behaviour isn’t just for firefighters — it’s a foundational step in planning safe, compliant developments in bushfire-prone areas. By recognising how fuel, weather, and slope interact, we can make informed decisions about where and how to build — and how to protect the people who live and work in these areas.

If you're developing a property and need clarity around bushfire risks, get in touch with our team at Bushfire Smart. We’ll help you understand your site, your obligations, and the best way forward.

• Fuel: Type, moisture, amount, and how it's arranged (e.g. shrubs vs trees).

• Weather: Wind speed and direction, temperature, and humidity.

• Topography: Fires move faster uphill—doubling in speed for every 10° rise in slope.

We use these factors to calculate a Bushfire Attack Level (BAL), which determines the construction requirements needed to reduce bushfire risk.

To learn more about how these factors are used in BAL assessments, see our article: Understanding BAL Ratings: What It Means, How It's Calculated, and What to Do Next.