BOM forecasts heavy rain for parts of NSW, SES urges residents to prepare

The SES says residents should be prepared for flash flooding when the rain arrives on the weekend. (ABC Central West: Mollie Gorman)

Parts of northern New South Wales that flooded twice earlier this year are forecast to miss the heaviest rainfall from a weather system that could bring "a month's worth of rain in a few days" to some parts of the state, the Bureau of Meteorology says.

Senior meteorologist Dean Narramore said a widespread rain event would move across much of the east coast, but Sydney, the Hunter and the Illawarra would see the highest falls.

"This time of year is normally a drier time of year than than summer, but we're still looking at probably widespread falls anywhere from 100 to 200 millimetres for large parts of the NSW coast over the four or five-day period from Saturday," he said.

Mr Narramore said the rain had started in the Top End – in the middle of what was supposed to be the dry season – and would move down through Queensland and northern NSW. 

"That's thanks to a low pressure system that's going to be situated across our eastern inland for a couple of days," he said.

"That's going to combine with some tropical moisture streaming down from the north to bring a really wet week to much of eastern Australia."

The rain is expected to hit saturated soils and brimming water catchments. (Supplied: WaterNSW)

Mr Narramore said the rain was unlikely to be as intense as earlier in the year, but noted that it would  be falling on already saturated soils and full water catchments.

"Multiple days of moderate to at-times heavy rainfall is likely to lead to some minor and moderate flooding," he said.

Mr Narramore said the Northern Rivers and south-east Queensland were likely to avoid the worst of it, but said falls of 50 to 100mm were possible in those areas.

The Nepean River is still swollen from the rain earlier this year, though not to the extent seen here. (ABC News: Brendan Esposito)

'Won't take very much'

NSW State Emergency Service spokesman Adam Jones urged residents to take care in the unpredictable weather.

"With the dams as full as they are and ground still quite wet it won’t take very much rain to cause flash flooding," he said.

"It’s important that everyone prepares if they’re in the areas that might be affected, including making sure their emergency plan is ready, making sure their emergency kit is ready.

"Make sure you limit any travel during the event, including driving to the conditions, only driving during severe weather when it’s essential and never drive through floodwater."