From lawnmowers to kids: How do Australia's bushfires start?

Katherine Chatfield
·Columnist
·8-min read

Although bushfires that are started by arson are widely reported, there are a number of other much more common causes.

For example, lightning – the majority of fires that burned during the 2019-20 Black Summer were caused by lightning.

“The old fireys joke goes; ‘There are three main causes of bushfires: men, women and children’,” says Roger Underwood, chairman of practical bushfire specialists, Bushfire Front.

“It’s true that many bushfires are started by humans, either accidentally or deliberately or stupidly.”

Here, we take a look at the causes of bushfires, and what can be done to prevent them.

A firefighter works to extinguish a fire in December last year in Western Australia. Source:
Nine different ignition sources have been identified by the Australian Institute of Criminology. Source:

Can bushfires start naturally?

Yes.

The majority of ignitions in remote areas are caused by lightning strikes,” Adjunct Associate Lecturer in Wildfire Investigation with Charles Sturt University, and Director of International Wildfire Investigation Consultancy, Wildfire Investigations and Analysis, Richard Woods, said.

“Positive lightning strikes only make up 10 per cent of all strikes, but they have a high potential to cause a bushfire.

“If lightning strikes a tree or log, it may start a fire immediately or remain benign, smouldering for weeks if not months until the fire danger increases and the revived fire spreads to surrounding vegetation.”

Sun rise during bushfire season in Australia.
The sun rises over bushland on the eastern seaboard of Australia. Source: AAP

What causes the majority of bushfires?

There are nine recognised categories for the ignition sources of bushfires, according to research by the Australian Institute of Criminology.

These are:

Smoking

Yes, cigarettes can cause fires, but conditions for a successful ignition must align.

“When someone discards a cigarette on the ground, there have to be quite specific conditions for a fire to start,” Mr Woods said.

“The cigarette needs to have 30 per cent of the burning tip in contact with the fuel [fine matted grass], there has to be low humidity of around 22 per cent or less, an air temperature of at least 26C, and low fuel moisture content, generally less than 14 percent.

“Importantly, there also needs to be wind influence at ground level.”

Man holding a cigarette while at the wheel of a car
Yes, cigarettes can cause fires, but conditions for a successful ignition must align. Source: AAP

Despite this, you should never throw a lit cigarette on the ground.

“There’s always the potential for a fire and cigarettes have been found to have caused bushfires,” Mr Woods said.

“It’s important to remain vigilant and for smokers to act responsibly.”

He adds, that while people assume most roadside ignition sources are due to cigarettes, vehicles are also responsible for these fires.

“For example, carbon can build up in a diesel vehicle exhaust and cause a fire,” he said.

Burning off

Agricultural burning off and land clearing can lead to accidental fires, Mr Woods said.

“It can be an issue if landholders aren’t well versed in managing fire and don’t prepare adequate fire breaks, have insufficient suppression equipment; or they ignore the weather forecast,” he said.

“For example, burning a pile of logs without adequate safeguards can result in fire escaping, sometimes many weeks after the main pile has burnt down.

“Residual heat can lead to an escape when high winds occur, which can result in embers being carried into surrounding grassland and starting a fire - often when the landholder assumes the fire has been extinguished.”

NSW Rural Fire fighters establish a backburn in Mangrove Mountain, New South Wales
Agricultural burning off and land clearing can lead to accidental fires, Mr Woods said. Source: AAP

Railways

“Heritage steam locomotives pose a risk when the fire danger rating is heightened as a result of hot windy weather.

“Escaping carbon from coal-fired locomotive exhausts may land in dry grassland fuel, then ignite. This is the reason most heritage tourist railways don’t operate on days of Total Fire Ban,” Mr Woods said.

However, modern locomotives can also pose a fire risk.

“Diesel locomotives can also eject fragments of carbon which can land in grass as far as 14 metres away from the rail line,” he said.

“Railway causes are also related to a maintenance issue; failure of brake shoes can result in ignitions on railway easements if they land in dry vegetation fuels.”

A campfire at the Darling River at sunset in the NSW outback town of Bourke
Many National Parks only permit cooking fires in designated areas, but even then, it’s important to be careful. Source: AAP

Campfires

Many National Parks only permit cooking fires in designated areas, but even then, it’s important to be careful.

“It’s common for bushwalkers to attempt to extinguish their campfire by just smothering it with soil,” Mr Woods said.

“But this creates an oven effect, retaining heat under the layer of soil. Then if the crust of the soil breaks down, the coals can ignite and the fire may then escape, particularly if it is impacted by strong winds.

“The best way to extinguish a campfire is to break up all remaining coals, drench it with water and check for any residual heat.

“Only then is it safe to cover with soil. Ideally, seek advice from the National Parks or Fire Service officers on where and when campfires can be lit,” he said.

Specific restrictions also apply during the Fire Danger Period and no solid fuel fires are allowed during a Total Fire Ban.

Man mowing his lawn
Slasher mowers used to mow grass can cause fire when the metal blades strike rocks resulting in sparks, Mr Woods said. Source: AAP

Equipment that can cause bushfires

“The use of equipment in bushland areas can be a source of ignition for a bushfire,” the analyst told Yahoo News Australia.

“For instance, the steel tracks of bulldozers can grate against rocks and cause sparks, slasher mowers used to mow grass can cause fire when the metal blades strike rocks resulting in sparks landing in dry grass, and diesel engines cause ignitions when hot carbon is ejected from exhaust systems.

“Using any powered equipment has the potential for the slightest spark, so operators need to be particularly vigilant in dry conditions.”

Seek advice from local fire agencies on the safe use of equipment during the Fire Danger Period.

Children light sparklers during New Year celebrations. Source: Getty
Children light sparklers during New Year celebrations. Source: Getty

Children

“There’s no malicious intent, but children can be curious about fire which leads to experimentation. They are often not aware of the potential danger of playing with fire in reserve areas,” Mr Woods said.

“The key is to ensure they are made aware of the danger in playing with fire, and that they don’t have access to matches or cigarette lighters.”

Lightning can cause bushfires

The majority of significant bushfires in Queensland, NSW, South Australia and Victoria during the 2019-20 bushfire season were caused by lightning.

“When lightning strikes vegetation, and there’s insufficient or no rainfall accompanying these storms, it can result in ignitions,” Mr Woods said.

“This is a big risk for fire agencies, as it’s often a challenge for ground resources to access these remote ignitions rapidly. Often they need to deploy firefighting aircraft as the first step to containing these fires in their early stages,” he said.

Lightning strike in the late evening with strong wind and heavy rain.
The majority of significant bushfires in Queensland, NSW, South Australia and Victoria during the 2019-20 bushfire season were caused by lightning. Source: AAP

Miscellaneous

“Powerlines have been responsible for causing bushfires on days of heightened Fire Danger Rating,” Mr Woods said.

“High winds can cause conductors to clash, trees to impact on conductors, or cause the failure of powerline hardware. This results in arcing and molten remnants coming into contact with vegetation, resulting in ignitions.”

Arson

Deliberate lighting of wildfires is a worldwide problem.

Malicious actions of an arsonist can result in significant risk to the community, threaten lives and cause damage to infrastructure and the environment.

However, as Mr Woods points out, Fire and Police Agencies are now better trained and prepared to investigate fires and identify those responsible.

“There has been a marked increase in the number of arsonists being identified and prosecuted,” he said.

However, the issue is ongoing, and the community has a key role in providing information to authorities to help prevent this criminal act.

A man walks past an intentionally set fire
Deliberate lighting of wildfires is a worldwide problem. Source: Getty

How many bushfires are started by arson?

Around 13 per cent of bushfires in Australia are deliberate, according to Australia's National Centre for Research in Bushfire and Arson.

In the 2019-20 bushfires, around one per cent of the land burnt in NSW was attributed to arson, and even less in Victoria.

“I can confidently say the majority of the larger fires that we have been dealing with have been a result of fires coming out of remote areas as a result of dry lightning storms," NSW Rural Fire Service (RFS) Inspector Ben Shepherd said during the height of the Black Summer.

That doesn’t mean the one per cent should be overlooked though.

“Arson is over-rated in terms of numbers, but not in terms of impact,” Underwood said.

“A skilled arsonist can play havoc, especially as they often pick a remote spot and light several fires.”

The sun rises over bushland on the eastern seaboard of Australia. Source: AAP
Bushfires can be ignited at any time if the conditions are right. Source: AAP

Can bushfires start at night?

“Yes. Bushfires can be ignited anytime when the fuel (grassland/forest) is in sufficient quantity, has lower moisture content and the weather (temperature, wind and humidity) results in a heightened Fire Danger Rating,” Mr Woods said.

What should I do if I see something that could start a fire?

“Fire and Police agencies rely on the eyes and ears of the public to help them in detecting bushfires and in providing any information regarding the cause,” Mr Woods said.

“And don’t assume your information is irrelevant. All States and Territories in Australia also use Crime Stoppers to assist in identifying those responsible for lighting fires. If you see something report it.”

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