Some 130,000 households were told on Thursday afternoon the volume of water surging out of the dam won't be the same as seen during devastating floods in 2021.

About 600 gigalitres of water had been predicted to spill, but with less rainfall in the catchment area than expected, WaterNSW announced the forecast had been revised down to between 300 and 350 gigalitres a day.

That's below the 440 gigalitres a day that surged out of the dam during flooding last year.

"This prediction is still subject to advice from the Bureau of Meteorology and will be ultimately determined by the intensity of the rain event and the inflows generated," WaterNSW said on Thursday.

Most dams in Greater Sydney have spilled, including all dams in the Upper Nepean region.

While Warragamba dam spillage has been revised down, several rivers are still above their major flood level or approaching it.

Moderate to major flooding is expected in western Sydney along the Hawkesbury, Nepean and Georges rivers, and the Colo and Macdonald rivers at Menangle, North Richmond, Penrith and Windsor, with dozens of suburbs on high alert.

The SES responded to more than 2500 requests for assistance and conducted 144 flood rescues in the 24 hours until 5pm on Thursday.

Further severe weather is expected around NSW, particularly along the coast.

The BOM says the slow moving offshore low pressure system that has delivered hundreds of millimetres of rain and floods across two states over the past week will begin to weaken.

Bureau meteorologist Jackson Browne says the low is expected to wash out overnight, but another cold front on the way from Victoria will bring more rain and severe weather into the weekend.

Tens of thousands of people living in Sydney's west, northwest and southwestern suburbs including Windsor, North Richmond, Camden and Milperra have been ordered to flee with evacuation centres set up in suburbs including Canley Vale and Blacktown.

Charity Save the Children Australia has set up child-friendly spaces at the Canley Vale centre and has also sent teams to northern NSW centres.

More than 300 schools across the state are closed because of flooding.

Endeavour Energy is advising customers in low-lying areas to prepare now for power cuts ahead of rising floodwater levels.

Some 650 properties - mainly along the Hawkesbury, Nepean and Colo rivers - have had power cut.

They are likely to be without power for several days.

Further north, thousands of properties remain without power after flooding earlier in the week.

Essential Energy said on Thursday night there were 13,800 customers without power in the Mid North Coast and Northern Rivers regions, after power had been restored to more than 28,800.

Much of the damaged network is still underwater.

In Lismore, the clean-up and search of flooded homes continues, with four fatalities confirmed so far.

The town is also facing shortages of essentials such as food, fuel, fresh water and cash as ATMs run out of money.

The BOM has warned areas on the Mid North coast and Hunter region already impacted by flooding could see renewed river rises with thunderstorms forecast across the district on Thursday afternoon.

Abnormally high tides and damaging surf warnings are also in place for practically the entire NSW coastline, with east-facing beaches at greater risk.

Coastal erosion is likely and low lying areas could also be affected by tidal flooding.

SES members have come to NSW from Victoria, South Australia and Tasmania to assist local units.

Insurers have already received more than 60,000 claims from recent flooding in southeast Queensland and around NSW.

The majority of those claims are from Queensland, however, the Insurance Council of Australia expects NSW claims to increase as flooding continues.

More than 80 per cent of the claims are for flooded properties, with the rest for vehicles.

The ICA predicts about $900 million worth of claims to come through insurers.

Australian Associated Press