The state's coast was drenched over the weekend, while Sydney recorded its heaviest rain in three decades and was battered by gale-force winds.

A massive clean-up is underway as emergency services continue to clear fallen trees, remove debris and extract cars stranded in floodwaters.

A man on Tuesday morning was rescued after clinging to a tree for almost 12 hours to escape floodwaters north of Bega.

Tens of thousands of households are also without electricity due to fallen power lines.

A flood watch warning has been issued for the bushfire-ravaged South Coast with high tides increasing the risk in low lying coastal areas.

Flood warnings are also in place for Hawkesbury River at North Richmond and communities around the Colo River.

The Bureau of Meteorology has issued severe weather warnings for the NSW coast from the northern rivers region down to the South Coast with abnormally high tides expected on Tuesday morning.

The bureau warned water levels during the high tide may cause inundation of areas including Coffs Harbour, Port Macquarie, Sydney, Wollongong, Batemans Bay and Eden.

Up to 550 millimetres of rain fell across many parts of the northern rivers, mid-north coast, Central Coast, Sydney, Blue Mountains and the Illawarra over the weekend.

Brogers Creek in the Illawarra region copped 433mm of rain in the 24 hours to 9am on Monday, while Wattamolla in Sydney's south received 418mm and Robertson in the Southern Highlands was soaked by 397mm of rain.

More than 15,000 calls for help were made to Fire and Rescue NSW over the weekend - which the emergency service says is more than three times the calls made during the peak of the bushfire season.

The NSW Farmers association says rainfall in the state's north has boosted soil moisture and is a "huge relief" for many livestock producers.

"The psychological boost that this rain has provided is vitally important," the organisation's president James Jackson said.

Les and Laura Jones' paddocks at Goolhi, near Gunnedah, have turned a lush green with the couple getting more than 60mm at their farm since Thursday.

But Ms Jones told AAP it would be a "false green" if they didn't receive more rain.

The BOM says it will take prolonged follow-up falls to break the drought.

The heavy rainfall on the coast was not good news for oyster farmers, with ash and sediment run-off from the recent bushfires expected to affect water quality.

Sydney's parched dams have had a boost with Warragamba Dam now at more than 60 per cent capacity after jumping from 43 per cent a week ago.

Australian Associated Press