The approaching weather system is “half the size of New South Wales”, a Bureau Of Meteorology meteorologist told NCA NewsWire.

Experts are predicting severe thunderstorms and flooding, and emergency services are preparing for evacuations, flood rescues, severe damage to homes, and for some communities to be potentially isolated by floodwater.

“The heaviest rainfalls we are expecting will be for the southeast of the state,” BOM incident manager Agata Imielska told reporters on Friday.

“In the Bega area, we are expecting rainfall accumulations of 200 to 300 millimetres, so very heavy rainfall.”

A flood watch for moderate to major flooding has been put in place for the area.

Warnings are also in place in northern NSW, where heavy rain could turn into thunderstorms, and residents in fire-hit areas are warned to be on high alert.

“When fires go through, we can see less vegetation (in the area), and that means soil is more likely to be eroded when heavy rainfall comes through,” Ms Imielska said.

“This can produce more flash flooding, as the rain doesn’t really soak into the landscape, it just runs off.”

Ms Imielska said this could be particularly dangerous where hilly or mountainous landscapes were present, or large trees, as their roots were not as embedded as they would be in areas that were not decimated by fire.

In Sydney, only about 10 millimetres of rainfall is expected but not to the extent of the south coast or northern inland areas – though meteorologists say this weather system is so intricate, anything could happen.

NSW State Emergency Service assistant commissioner Greg Lynch said crews were preparing communities for evacuations.

“We have additional flood resources prepared and ready to go, to the south coast in particular, and we’ve also been looking at having other out of area support from our partner emergency services,” he said.

Mr Lynch said the SES had called in helicopters in the Bega Valley and into the Moruya area.

“Helicopters will support us with flood rescue or with resupply if the case is that a number of communities or areas within the south coast become isolated,” he said.

“The State Emergency Service are preparing for evacuations and we are asking communities to prepare for evacuations – to have their plans ready if the call comes.”

Mr Lynch said anyone living in these areas should clear their backyards and try to avoid leaving their homes.

“Unless you have to go out, please stay at home,” he said.

Reflecting on similar storms seen about two weeks ago, the SES boss issued a strong call for people to exercise common sense as the storms thundered in.

“People need to take responsibility, they need to ensure they listen to the messaging we’re providing them and that message is clearly, do not drive, walk, or ride through flood waters,” Mr Lynch said.

“I think on our last occasion we rescued more than 30 people from flood waters. That’s putting themselves at risk, and also putting those who have to go and rescue him at risk.”

Meteorologist Dean Narramore said earlier on Friday moderate to heavy rain would also impact southwestern Queensland.

“Some areas will receive their monthly August average in just one day,” Mr Narramore said.

More than two thirds of Queensland is still officially in drought and the rain will bring a reprieve to struggling farmers and graziers.

The bureau said the low would produce showers, rain areas and thunderstorms, possibly severe in parts, over southern Queensland.

The trough system will move off the east coast during Saturday, with cooler and drier air spreading over most of the state as a new ridge develops over central Queensland.

Squalls have already moved through Brisbane’s southern suburbs and on to the Gold Coast.

Some severe storms were likely about the southeast inland and central highlands. Moderate to heavy rainfall is possible in the far south of the Maranoa and Warrego district.

Daytime maximum temperatures will be below average in the west, near average for central and eastern parts. Mostly light winds over the northern tropics, moderate to fresh northeast to north-westerly winds over the south.