Heavy falls, floods predicted for NSW

© iStockphoto.com/Carmen Martínez Banús

ASSISTANCE crews are on standby as rain has started to develop in NSW ahead of forecast heavy falls which could bring floods.

A low pressure system has reached the state as a result of Tropical Cyclone Laurence which has plagued northern parts of Western Australia in recent weeks.

"We've had some rain move across the southern districts, the ACT and the south coast," Bureau of Meteorology forecaster Jake Phillips told AAP on Friday morning.

"This rain we've seen going across the southeast is likely to be pretty trivial compared with what we are expecting over the north and central inland over the next few days."

There are forecasts for heavy downpours in some areas which could bring more than 100mm of rain in less than a day and may cause flooding.

The areas likely to be affected by heavy rain are inland from as far north as the Queensland border, south through Bourke, Nyngan and Dubbo and as far east as Orange.

"We are expecting the falls to be well over 100mm in those areas in the next couple of days," Mr Phillips said.

Some 30 State Emergency Service (SES) crews are on standby in some of the areas ahead of the rain, ABC Radio reported on Friday.

Rain is also expected in Sydney some time after midday (AEDT) Friday.

"There's not going to be any huge fall in Sydney today but moving into tomorrow it looks like it will pick up a fair bit... and won't clear until Monday," Mr Phillips said.

"We are not expecting that it will be enough in Sydney to cause any major problems... but it still could be a pretty decent fall."

The BOM has issued a severe weather warning for the central west slopes and plains and in upper western NSW ahead of the rain and possible isolated thunderstorms.

A separate flood watch has also been issued for the Castlereagh, Bogan, Lower Namoi, Lower Macquarie, Barwon, Paroo and Warrego Rivers.

On Thursday, Emergency Services Minister Steve Whan said more than 200mm of rain was expected to fall by the end of the weekend in parts of the state.