The city set to cop up to 120mm as rain lashes large parts of Australia

NSW should brace for a drenching this weekend, with rain expected to linger across the state right through until late next week.

While Sydney can expect a chance of showers on Friday afternoon, over the weekend it is near certain there will be rainfall, with showers increasing on Saturday, dumping potentially 3 to 10mm.

The Bureau of Meteorology (BoM) said there is a "near 100 per cent" chance of rain on Sunday, with 35 to 60 mm of rain forecast and the likelihood of thunderstorms during the day.

From Monday through to Thursday, there is either a medium to high chance of rain at this stage in Sydney with about 50mm set to fall - bring the seven-day accumulated total to a potential 120mm.

Flooding risk remains across NSW

While the wet weather might put a dampener on weekend plans in the state's capital, the weather could be potentially disastrous in other parts of the state.

Several regions in the state's west are already flooded from heavy rain during the week.

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BoM NSW hazard preparedness response manager, Jane Golding, says the system is likely to deliver widespread rain to parts of the state as well as thunderstorms, at a time when the landscape is "quite sensitive".

"We've seen quite a lot of rain over the last 18 months... the catchments are saturated, quite a lot of dams are full and some of our rivers are still quite high," Ms Golding says.

Flooding destroys crops in central west

On Thursday morning, the Lachlan River peaked at Forbes at 10.53 metres and resulted in some flooding of houses and destroying crops.

The peak is just below the September 2016 flood level, though so far, the damage has not been as severe as it was back in 2016 when hundreds of homes in the area were inundated.

Cottons Weir has experienced major flooding, while Jemalong is in for the same on Friday, with minor to moderate flooding likely at Condobolin later next week.

Some growers in the area have tried desperately to salvage any crops they could before more wet weather hits this weekend.

On Thursday disaster assistance was made available by the state and federal governments across several local government areas from the Blue Mountains, to Forbes and Moree.

The Bureau of Meteorology still has a "moderate to major" flood warning in place for the Lachlan River while the Bureau warns flooding is likely at several inland rivers in the central and northwest of the state.

The catchments likely to be impacted by flooding include:

• Upper Macintyre River — minor flooding

• Gwydir River — minor flooding

• Peel River — minor flooding

• Namoi River — minor to moderate flooding

• Castlereagh River — minor flooding

• Macquarie River to Bathurst — minor flooding

• Orange, Molong and Bell River — minor to moderate flooding

• Belubula River — moderate flooding

Forbes Mayor Phyllis Miller said the whole area continues to sit on a "knife's edge."

"We're already experiencing major flooding ... if we get more rain here in the catchment its going to cause real grief."

The SES has advised residents and businesses to keep their flood preparations in place with more rain forecast that may contribute to increased flooding and river rises.

Nullarbor cops biggest monthly drop on record

It's not just NSW which will see a wet weekend, heavy rainfall is expected all across southern Australia.

"A cloud band has thickened across Western Australia, South Australia and parts of Victoria and New South Wales," BOM Meteorologist Jonathan How said.

He added the Nullarbor has had its wettest November since records started some 130 years ago, with 85mm so far.

Showers over the Nullarbor are expected on Friday and Saturday. It will be partly cloudy on Sunday before a shower or two on Monday and Tuesday.

A late shower or two is expected on Friday in Adelaide, with a little more rain expected on Saturday, before clearing up.

Melbourne and Brisbane weekend forecasts

There's only a slight chance of a shower on Saturday for Melbourne, with a 40 per cent chance of up to 2mm falling on Sunday.

However, minor flood warnings have been issued for the Latrobe River and the Thomson River.

Brisbane has a sunny forecast for the week ahead, but the southern part of the state is not exempt from the rain expected across the border in NSW.

There are nine current flood warnings in Queensland, according to the BOM.

The catchments of concern in Queensland are:

• Moderate Flood Warning for the Balonne River

• Moderate Flood Warning for the Dawson River Catchment

• Minor Flood Warning for the Barcoo River

• Minor Flood Warning for the Bulloo River

• Minor Flood Warning for the Georgina River and Eyre Creek

• Minor Flood Warning for the lower Warrego River (QLD)

• Minor Flood Warning for the Paroo River (QLD)

• Flood Warning for the Diamantina River

• Final Flood Warning for the Macintyre Rivers

Mr How said a low pressure system will deepen on Saturday and ultimately the position of the low will determine how far north the rain stretches in Queensland over the coming days.

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