It has become a familiar and reassuring sight — planes and helicopters dumping loads of water and fire retardant on out-of-control blazes.

Key points: After this week's catastrophic fire levels, experts are asking if we have enough firefighting aircraft available year-round

After this week's catastrophic fire levels, experts are asking if we have enough firefighting aircraft available year-round Large aircraft and helicopters are expensive to lease, so firefighters borrow aircraft from the northern hemisphere when needed

Large aircraft and helicopters are expensive to lease, so firefighters borrow aircraft from the northern hemisphere when needed Fire seasons are overlapping between the northern and southern hemispheres, so aircraft may not always be available

But experts are warning that fire services do not have enough large aircraft to deal with the unprecedented magnitude of fires we are now faced with.

Former NSW Deputy Fire and Rescue commissioner Ken Thompson said Australia needed to invest in more large aircraft that are available year-round.

"We've got real concerns about the impact that climate change is having on fire behaviour and we've got very serious concerns about the small numbers of large aircraft that are available to support firefighters and local communities," he said.

"What we really need is the large, high-volume water bombers that can deliver really large amounts of water very quickly to serious fires."

Australia's fleet

There are about 160 aircraft available to fight fires in Australia.

They include small planes and helicopters used for command and control, through to larger fixed-wing planes and heavy helicopters that can dump thousands of litres of water.

The National Aerial Firefighting Centre controls the fleet of aircraft shared between the states and territories. ( Supplied: NAFC )

Mr Thompson said heading into catastrophic fire conditions small aircraft simply could not cover the same amount of ground.

"At the moment, the smaller aircraft can only carry around 1,500 to 2,000 litres. These [larger] aircraft can carry up around 15,000 litres," he said.

"Which is the type of capacity you need to be able to knock down some of those incredibly large fires that we're seeing now, that we didn't see 10 or 15 years ago."

Mr Thompson is part of a group of 23 former fire and emergency leaders who wrote to Prime Minister Scott Morrison in April warning him that Australia needed to resource large water bombers.

"This is an important enough issue that the Prime Minister needs to become involved," Mr Thompson said.

"But we just haven't been getting the response that we would have hoped for from him."

Stretching resources

Most of the large-scale aircraft in Australia are borrowed from the northern hemisphere and, as fire seasons begin to overlap, Mr Thompson said sharing those resources was getting harder.

This year, nine large helicopters and seven fixed-wing aircraft have been contracted from the northern hemisphere, but only half have arrived in time for NSW and Queensland's devastating fires.

"We typically don't get access to these aircraft until about December and even now I think that that's going to be problematic because of the size of the extended fire season in the northern hemisphere," Mr Thompson said.

California has one of the largest fleets of firefighting aircraft in the world. ( Reuters: Kyle Grillot )

Firefighting aircraft are managed by the National Aerial Firefighting Centre (NAFC), a cooperative body funded by both State and Territory Governments and the Federal Government.

As ABC's Background Briefing revealed in October, two years ago the NAFC sent a proposal on behalf of all states and territories to Canberra for an annual increase of $11 million above its existing $15 million in funding, but they still do not have an answer.

In a submission to a senate inquiry into the 2016 Tasmanian bushfires, NAFC said:

"The Australian Government funding is, however, forecast to diminish in real terms, whereas the cost of providing aerial resources will rise. This may lead to a reduction in access to aerial resources in the future."

General manager of NAFC, Richard Alder, said while large water bombers were useful they were not a silver bullet.

"I think if you asked any firefighter, any aerial firefighter, we would probably always like to have more," he said.

"But certainly the fleet that we've got available to us is quite comprehensive and does provide very valuable support to the firefighters on the ground."

Mr Alder agreed that southern and northern hemisphere fire seasons overlapping was a concern.

"It's causing a few nervous moments. So far we've been able to manage that," he said.

Is bigger better?

Senior researcher at CSIRO's Department of Bushfire Behaviour and Risks, Matt Plucinski, said aircraft had a number of advantages for fighting fires, but they were most important in the initial attack.

"They can access fires much more quickly because they can travel more quickly and in a straight line," he said.

Firefighters receive a briefing during the September 2019 fires close to Dorrigo, NSW. ( ABC News: Meghna Bali )

"They can access fires when they're in difficult terrain that might be hard to get to or unsafe to get to on the ground."

But Mr Plucinski said there was not enough research to understand the most effective use of large aircraft and what Australia might need in the future.

"To get people on the ground to measure it at the right time, to not be involved, and to not get in the way is really difficult to organise," he said.

"Particularly periods like this and [the firefighters] have got their heads down, working to put the fires out and probably don't have much time to think about anything else at the moment."

Costs-benefit analysis

The Boeing 737 contracted by the NSW Rural Fire Service can carry 15,000 litres of water or fire retardant. ( Supplied: Rob Miller/Coulson Group )

The biggest firefighting aircraft in Australia at the moment is a Boeing 737 water bomber, leased by the NSW Rural Fire Service (RFS) and available year-round — known as 'Gaia'.

A DC-10 air tanker from the United States was also on its way to join its fleet and can drop 35,000 litres of water or fire retardant in eight seconds, and will be in the country for the rest of the bushfire season.

But they do not come cheaply.

It costs about $2.5 million a year to lease a large fixed-wing airtanker and about $2.2 to lease large helicopters.

Sorry, this audio has expired State of emergency

Mr Plucinski said work needed to be done on the sums, to work out what the real need for large aircraft was, and whether or not the cost was warranted.

"On a day like the catastrophic day the other day, you could never have enough of them, although you'd logistically be challenged to operate them all," he said.

"But then on other days you probably don't really require much at all, so there's a balance."

And when conditions get too bad, not even giant water bombers can make a difference, with extreme weather and fire storms grounding them.

"All suppression is less effective in more severe conditions just because the fire behaviour is too intense to allow effective suppression," Mr Plucinksi said.

"In those sorts of situations most resources, including aircraft, drop back to protective roles … they're protecting things like houses and communication towers that are in the way of the fire."