NSW has been smashed with heavy rain, wild winds and freak waves, with authorities warning people in Sydney and the state's south the worst is yet to come.
Several low-lying suburbs in Sydney's southwest and parts of its north were evacuated as between 70 and 90mm fell from the grey skies blanketing the sodden city on Sunday.
"Sydney is seeing some of the heaviest totals," a Bureau of Meteorology spokesman told AAP.
Homes at Narrabeen are being evacuated with the help of the NSW Fire and Rescue service, while residents in a nursing home at Canley Vale are also being helped to leave.
"We have been very busy," FRNSW commissioner Greg Mullins said.
"There have been times this weekend when every single crew within the greater Sydney area was deployed."
Further west, the heavy rain has caused the Nepean and Georges Rivers to swell to dangerous levels, prompting authorities to evacuate locals.
Almost 526mm of rain has fallen in the Nepean River's catchment area during the past two days, with 128mm coming during the past six hours of intensified downfall.
"This forecast rain is expected to cause major flooding on the Upper Nepean River at Menangle, and minor to moderate flooding downstream," the SES said.
There is a possibility the river could peak at 3.9 metres in Penrith on Monday morning, but it's not expected to cause significant damage or impact local residents.
The main concern around the Sydney region is the highest tide of the year, due Sunday night, combining with five to seven-metre waves and destructive 125km/h winds.
"It's certainly not safe to be down at the beach," the bureau spokesman told AAP.
Earlier on Sunday, a freak 12m wave was detected at Botany Bay, prompting authorities to issue warnings about erosion and sea-water flooding for those living close to the coast.
Meanwhile, SES crews are searching floodwaters at Leppington over concerns a car has been swept away.
The foul weather smashed Sydney and surrounds after it moved down from the northern coastline on Sunday.
The state's north was also hit hard, with a "phenomenal" 400mm of rain hammering Coffs Harbour over the weekend, the BOM spokesman said.
Flood fears have also prompted an evacuation order for locals at North Lismore, while residents in areas around Tweed and Tumbulgum are on watch for potential evacuations due to local river flood warnings.
During the weekend's powerful storms about 7700 calls for help were made to the SES, who have performed about 200 flood rescues.
More than 50 were underway in Sydney alone on Sunday evening.
At least 85,000 homes and businesses were still without power on Sunday night.
The storms have battered their way down the NSW coast but there's another day of savage weather and heavy rain expected in the state's south on Monday.
A further 100mm of rain is expected to fall on the Illawarra, which has in parts already copped 154mm.
Motorists are advised to avoid all non-essential travel as flash-flooding continues to swamp roads.
The weather is expected to begin clearing around Sydney on Sunday evening as the system moves south towards Tasmania.