Emergency crews have rushed to 30 flood rescues in less than an hour as heavy downpours engulfed millions of residences along the east coast.
One of the many dozens of rescues included 30 passengers stuck on a light rail in Sydney, which was lashed with 120mm of rain.
Rainwater flooded homes in inner-city suburbs and seeped through roofs in Wollongong after three consecutive days of heavy falls across the central NSW coast.
At least two tornadoes were reported across the state, including one captured on camera from a fire tower at Young as it tore through a field in NSW's southwest.
The rain continued into Thursday, with Newport on Sydney's Northern Beaches recording 122mm in the day to 9am and Cronulla receiving 147mm.
It made September one of Sydney's wettest on record.
Multiple severe weather warnings remain in place, including alerts for damaging winds and hazardous surf across NSW, but the rain is slowly easing with sunny conditions predicted for Friday.
Huge swells at Bronte Beach in eastern Sydney drew in many avid swimmers as the sun peered through clouds.
However, State Emergency Services Assistant Commissioner Dean Storey urged NSW residents to remain vigilant, with storm season brewing.
"This is just the beginning of what we are expecting to be a very busy storm season and it's important people take simple steps to prepare," he said.
The SES received more than 850 calls for assistance in 24 hours, mostly due to heavy rain.
There was a surge in calls from across Sydney for 50 minutes on Wednesday when people became stranded in their homes or trapped in vehicles on flooded roads.
A group of hikers was rescued at Wattamolla, south of Sydney, after heavy rainfall on Wednesday afternoon.
Wind gusts with peak speeds of 90km/h were expected to last into the afternoon on Thursday due to a complex low-pressure system, the Bureau of Meteorology said.
The low-pressure system has now moved further offshore, easing conditions throughout the state.
Large and powerful south-to-southeast waves are forecast to batter the northern and central coasts through to early Friday, bringing hazardous surf and increased risk of coastal erosion.
Strong wind warnings have been issued for the Byron, Coffs, Illawarra Batemans and Eden coasts with hazardous surf warnings for the Macquarie and Hunter coasts.
NSW Police Marine Area Command has advised people to consider staying out of the water and avoid walking near surf-exposed areas.
