Sydney Storm Update: Everything You Need To Know Before Ducking Out For Lunch

One person is dead, two police officers have been injured and almost 130 flights cancelled after a month's worth of rain was dumped on Sydney in just two hours.

A series of thunderstorms battered the NSW coast from Newcastle down to Wollongong on Wednesday morning, causing havoc for commuters and emergency services.

The weather bureau warns more rain and hail is likely later in the day, as authorities urged people to think about alternatives for their afternoon commute or school pick-up.

A person was killed in a crash on The Esplanade in Thornleigh, on the city's upper north shore, while a tree fell on two police officers who were trying to help drivers trapped on a flooded road in North Ryde.

A probationary constable suffered a suspected broken leg and has been taken to hospital in a serious condition, while her partner suffered a possible concussion and minor injuries.

"Delay your journey or work from home today - we want as less cars on the road," NSW Police Assistant Commissioner Michael Corboy told reporters.

"We do not want any further tragedies to happen today."

More than 118mm fell at Observatory Hill in Sydney's CBD in the six hours to 11am - easily eclipsing the November average of 83.8mm.

Mosman recorded 123mm while Chatswood had 119mm, including 66mm within one hour.

Shoalhaven's Porters Creek Dam has recorded the state's highest total so far, copping 168mm this morning.

A minor flood warning for the Cooks River at Tempe.

Sydney Airport has repeatedly closed one of its runways during the morning as it tries to operate amid strong winds and 45mm of rain.

About 130 domestic flights and four international flights have been cancelled due to the weather.

The NSW SES has 50 crews responding to 560 calls for help, having already rescued 12 people from floodwaters.

An SES spokesman told AAP their major concern was people driving into flood waters, putting their lives and the lives of volunteers at risk.

Roseville Bridge and dozens of roads across the city were shut due to flash flooding while a landslide on Spit Road in Mosman also caused issues.

An overflowing weir on the Parramatta River led to ferries being cancelled between Sydney Olympic Park and Parramatta, while rail services in the inner west were cancelled during to standing water.

More than 5000 properties are without power across the city, including the CBD and northern suburbs, with lightning strikes and strong winds damaging lines.

AusGrid said the torrential downpour flooded an underground substation at Wynyard.

A severe thunderstorm warning for heavy rainfall, damaging winds and large hailstones is still current for the Sydney metro, the Hunter and the Illawarra regions and parts of the Mid North Coast, South Coast, Central Tablelands, Southern Tablelands, North West Slopes and Plains and Northern Tablelands.

The heavy rain isn't expected to ease until early Wednesday evening, but damaging winds could hang around until Thursday.