Devastating storms have ripped though eastern NSW forcing flood evacuations and leaving leaving tens of thousands without electricity.
Utility companies were on Sunday rushing to restore power to for the more than 100,000 affected customers.
Communities along the Hawkesbury River northwest of Sydney have been ordered to evacuate ahead of surging floodwaters.
The NSW State Emergency Service is warning residents in parts of Pitt Town, Richmond and Gronos Point that flood levels along the Hawkesbury could be their worst since 1992.
Residents in low-lying areas near the Narrabeen Lagoon in northern Sydney were ordered to evacuate on Sunday night.
The southwest Sydney suburbs of Moorebank, Chipping Norton and Milperra were also ordered to evacuate.
The SES said people risked being trapped without power and water if they didn't leave these areas.
Emergency services have been swamped with calls since the deluge set in on Friday, while the extreme weather has caused transport chaos across Sydney.
"This wet and windy weather is really wreaking havoc on our roads today, with paramedics responding to five car accidents every hour since Friday night," NSW Ambulance spokesman Giles Buchanan said on Sunday afternoon.
"We've responded to multiple trees that had fallen onto cars, trees into houses and units, and people trapped in cars in floodwaters."
NSW Maritime says a number of boats have been sunk by the turbulent conditions at sea.
Four people were hospitalised on Sunday afternoon after a tree fell on their car in the Sydney CBD.
A 16-year-old boy has been taken to hospital with suspected broken ribs after he was trapped between debris in waist-deep water for two hours in the Hunter region.
The teen was rescued by emergency services after falling into Allyn River while canoeing at about 9am on Sunday.
Utility company Ausgrid says more than 77,000 customers have lost electricity across Sydney, the Central Coast and Newcastle.
Endeavour Energy says a further 26,000 customers are without power in Sydney, the Blue Mountains and Southern Highlands regions.
"Crews are reporting extensive damage after very strong winds brought down power lines in many areas," Endeavour Energy said.
A severe weather warning was issued on Sunday for the entire coast of NSW, and will remain in place on Monday.
The Bureau of Meteorology says the deluge could cause "life-threatening flash flooding" in the Hunter region, the Central Coast, greater Sydney, Illawarra and the Blue Mountains.
Sydney, the Central Coast and the Blue Mountains were soaked by between 200 and 400mm of rain from 9am Friday to 5pm Sunday.
The last time Sydney and the state's coast had as much rainfall was mid-2016.
"But we've surpassed those figures and you have to go back as far back as 1998 to see totals like we'll get," a weather bureau spokeswoman said.
NSW Police Assistant Commissioner Karen Webb urged drivers to take caution and avoid floodwaters.
"There are currently hundreds of calls for assistance, including trees, boulders or power poles down onto cars and homes, and across roads, as well as power outages and localised flooding impacting various roads and traffic lights," she said.
"I'm disappointed that I need to remind people to act responsibly and not to take risks in these types of conditions, especially when around floodwaters."
Dams around Sydney, including the major Warragamba Dam, are swelling to their highest levels in years.
Australian Associated Press