The heavens have reluctantly opened, bringing a few millimetres of rain to the scorched south-east, but authorities say it can actually hinder their efforts to fight bushfires.

Key points: A cool change has brought temporary relief for the fire-ravaged south-east, but rain can hamper firefighting efforts

Rain can prevent back-burning, making it harder to build control lines and lead to patchy burnt areas that can flare up again

For rain to extinguish the fires, it will require inches of steady falls over an extended period

Up to 15 millimetres was recorded over 24 hours across some parts of the fire zone. In East Gippsland, Bairnsdale recorded 12mm, Nowa Nowa 6mm and Buchan 5.2mm.

Some areas only got a sprinkling that left nothing in the gauges.

So what will it take to get these mega-fires out?

Greg Allen, spokesperson for New South Wales Rural Fire Service, said while the cooler conditions were welcome, rain was not a good thing in all cases.

"It can hamper efforts to put in back-burning, those tactical firefighting operations that we use at times to strengthen containment lines," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 18 seconds 18 s The fire which hit Corryong, Cudgewa, Tintaldra and Towong is likely to burn for weeks.

Work to clear fuel around the current fires to try and stop them spreading continues apace before conditions are expected to deteriorate again later this week.

According to Thomas Duff, a bushfire behaviour expert from the University of Melbourne, light rain can also make it difficult for heavy machinery to put in control lines.

"What [firefighters] do is drive around the perimeter of the fire and push away the fuel and make what they call a mineral-earth break that makes this line of dirt all the way around the fire.

"So you are stopping the fire by taking the fuel away."

The rain can hamper these efforts because it makes things slippery and difficult to operate the machinery, evidenced by a fire vehicle overturning on a slippery track in Victoria on Sunday.

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A small amount of rain can also be a nuisance because it can put out some of the fire but not all, resulting in patchy fires within the control lines.

"Having unburnt areas within your control lines is actually quite dangerous because they can be a source of new fires or spotfires when things dry out again," Dr Duff said.

So what will it take to put the fires out?

Dr Duff said the fires could stop if:

there was sufficient rain over a prolonged period to wet the fuels — this is potentially sooner around the Queensland border but could be months away in Victoria

there was sufficient rain over a prolonged period to wet the fuels — this is potentially sooner around the Queensland border but could be months away in Victoria there was a long period without strong winds that allowed firefighters to contain the fires

there was a long period without strong winds that allowed firefighters to contain the fires they ran out of forest to burn (and the fire coming out to grassland where it is easier to suppress)

What you really want is lots of steady rain, that isn't too hard, over an expended period.

Dr Duff said it was hard to say how much rain would be enough to stop the fires, particularly as the current rainfall deficit was unprecedented.

But we are talking inches of rain (one inch is around 25mm), not the couple of millimetres around at the moment.

There is rain around but it isn't going to be enough to put the fires out in the south-east. ( Supplied: Bureau of Meteorology )

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But a lot of rain can also be a problem.

Soils in very dry landscapes can become hydrophobic — water just slides off rather than soaking in — which can result in flash flooding.

Dr Duff said there was a fatality from flash flooding after major bushfires in 2003 which stretched from Canberra into northern Victoria.

When fire is being pushed by strong winds, very little can be done to stop it, but a lull in hot and gusty conditions can give firefighters a chance to douse the flames.

But Dr Duff said that would be a huge effort at this point.

"When you look at the length of perimeter that needs to be built, this is pretty unprecedented. It hasn't really been done in Australia before," he said.

"We're in the millions of hectares now, which is pretty unusual."

The length and breadth of the season is also a factor. In New South Wales, the fire season has been going on for months already, and Dr Duff said in Victoria the season could still go on for months.

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"That long period of time means it's a very difficult period and a huge amount of work for the firefighters to actually stay vigilant and stay active to be able to control the fire."

Then there could also be the chance that the fires burn into terrain where they are easier to fight, or into the ocean.

Fires in forests burn at a far greater intensity because of high fuel loads; if they moved into grassland, where there is less fuel, it would be easier for firefighters to get on top of them.

What's the forecast?

There is still no widespread rain on the forecast for southern Australia — there isn't even the chance of a good break in the immediate future.

Sadly, it's currently looking like there will be another peak in fire weather late this week.

In the north, things are starting to look decidedly more tropical and there is hope that the monsoon finally making its way down could break up the hot air that has been brewing over central Australia.

Cyclone Blake is predicted to impact the Kimberley and Pilbara coasts. ( Supplied: Bureau of Meteorology )

The major climate drivers have now returned to neutral conditions, meaning there is no big push to wetter or drier conditions than normal over the coming months.

But for southern Australia that is a bit of an empty statement — summer in the south is normally hot and dry.

Typically the fire season in South Australia and Victoria would only be just beginning, with the worst conditions coming late in summer.

Dr Duff said we wouldn't expect widespread rainfall in Victoria until the autumn break, which is typically expected around Anzac Day.