The weather is expected to improve on Monday with the bureau forecasting a medium chance of showers and winds of up to 35km/h.
The SES says its operations centre is being inundated with phone calls from residents all over the state requesting assistance.
It has received 5500 calls for emergency help and has performed 120 flood rescues, half of which involve stranded motorists.
The SES says most requests for assistance have been for sandbags, leaking roofs and trees and branches brought down by strong winds.
"We ask the public to please be patient as there are current delays in being answered," the SES said.
The SES Warringah/Pittwater Unit is offering sand and sandbags available for pick up at Narrabeen Beach Surf Club.
In Sydney's south a flood evacuation warning has been issued for low lying parts of Milperra, Chipping Norton, Picnic Point, East Hills, Carramar, Canley Vale and Lansvale. Evacuation centres have been set up at Liverpool, Cabramatta and Revesby.
A raft of public transport services have been cancelled and flash flooding is affecting roads throughout the metropolitan area.
'Avoid non-essential travel'
Rebecca Walsh, a spokeswoman from the Transport Management Centre said people should "avoid all non-essential travel".
"The best advice is to stay off the roads as it's very unpredictable out there," she said.
Ms Walsh said people who did have to travel should "drive to the conditions and allow plenty of extra travel time".
Weatherzone senior meteorologist Brett Dutschke warned high winds would continue throughout the day.
"We still have several more hours of intense weather to come," he said.
Up to 125mm of rainfall was recorded in the 12 hours to 10am in the Cooks River catchment upstream of Tempe Bridge and more heavy rain is forecast for the next six hours.
Residents near the river are advised to be on alert due to the severe conditions.
There is also a minor to moderate flood warning for the Nepean River, in Sydney's west with up to 285 millimetres of rain falling over the catchment in the 24 hours to 9am. A further 190 millimetres falling during the past six hours.
This rainfall is expected to cause moderate flooding in the Nepean River at Menangle, and minor flooding at Camden and Wallacia.
Further heavy rainfall, in the order of 100 to 150 millimetres, is forecast for the Nepean, Wollondilly and Hawkesbury River valleys in the next 12 hours.
Emergency crews are working to restore power after faults caused by the rain and wind and trees falling on powerlines.
Energy networks Ausgrid and Endeavour say more than 28,500 homes and businesses remain without power throughout Sydney and the Central Coast. There are power outages in Westleigh, Bass Hill, Yagoona, St Ives, Terrey Hills, Berowra Heights, Allambie Heights, Beacon Hill, Padstow, Punchbowl, Greenacre, Eastwood, Denistone, Brooklyn, Vaucluse, Bundeena, Zetland, Waterloo, Maroubra and Hillsdale.
The Bureau of Meteorology said large and powerful surf conditions were expected to be hazardous for coastal activities such as rock fishing, swimming and surfing. Beach conditions in these areas will be dangerous and people should stay well away from the surf and surf exposed areas, it said.
Nobbys Head recorded a 106km/h gust at 11.22pm on Saturday night while Fort Denison recorded a wind gust of 90km/h 20 minutes later. Norah Head recorded a 98km/h wind gust at 12.21 am and Kurnell recorded a 90km/h wind gust at 1.30am.
Sydney Morning Herald Online