While rainfall eased since the morning's deluge, conditions are expected to worsen again just in time for the trip home as the intense low over Sydney's west and south tracks towards the north and east.

"Due to the severe weather affecting Sydney, we’re asking customers to allow plenty of extra time travelling home this afternoon, and if possible, leave work early to avoid the peak," said Marg Prendergast, coordinator general at Transport for NSW.

A Bureau of Meteorology spokeswoman said a southerly surge is expected to push up the coast, which will bring heavy rain and thunderstorms back over the city within the next couple of hours.

Weatherzone's Tom Hough said the renewed rainfall surge was likely to hit in the middle of the evening commute – between 4pm and 8pm.

To make matters worse, the weather was still making an impact on public transport services, although the Transport Management Centre said the light rail between Central and Dulwich Hill was operating again after the line was closed earlier in the day due to flooding and fallen trees.

Ms Berejiklian visited the State Emergency Service headquarters in Wollongong on Wednesday afternoon, and urged everyone to use "common sense".

"We urge everybody to please take safety precautions, especially when going home this evening,” she said.

SES Commissioner Mark Smethurst said Wednesday had been the wettest November day for NSW since 1984 and the wettest day since 2015 on their records.

As of 2pm, SES crews had already attended to more than 1000 jobs across the state with many more call outs expected into the evening.

“Particularly as people leave school and work,” he said. “I urge everybody to take care as the winds pick up this afternoon, with that extra rain it’s going to be even more hazardous.

“It will be abating this evening and the winds easing off tomorrow during the day.”

Sydney was thrown into city-wide chaos on Wednesday as a month's worth of rain was recorded in a matter of hours, and is set to continue with a severe thunderstorm warning issued for much of the NSW coast.

In less than two hours, Sydney recorded its November monthly average rainfall, according to Weatherzone. Between 5.20am and 7am the Observatory Hill weather station recorded 84.6 millimetres of rain and by 11.30am the rainfall had ticked past 118 millimetres. The November average is 83.8 millimetres.

Flash flooding coincided with the morning commute, closing a number of major roads, and slippery conditions are certain to continue throughout the day.

The Bureau of Meteorology says severe thunderstorms, from the south coast to the northern rivers, could bring large hail storms and damaging winds on Wednesday afternoon.

The bureau issued the warning about 12.30pm, saying thunderstorms are likely to produce heavy rainfall that may lead to flash flooding and damaging winds over the next several hours and may affect Newcastle, Gosford, Sydney, Wollongong, Nowra and Katoomba.

Traffic and Highway Patrol Commander, Assistant Commissioner Michael Corboy said the conditions in the morning were some of the "worst" Sydney has ever seen, and urged people to work from home, as the wild weather makes road conditions dangerous.

He confirmed one person died after a crash at Thornleigh on Wednesday morning and two police officers were injured after a tree fell on them while they were helping motorists during flash flooding on Pittwater Road.

The officers were clearing the road about 7am when a tree fell on them and the vehicle. A female officer was taken to hospital in a serious but stable condition with a suspected broken leg.

The storms started to move over Sydney about 5.30am on Wednesday, with intense downpours, lightning and thunder hitting Sydney's west first, before moving east across the city to the coast.

The deluge in the city was so intense that water started pouring out of the walls at Woolworths at Town Hall.

Flooding caused all light rail services to be cancelled and replacement buses were not immediately available in the morning peak.

Transport for NSW also warned regular bus passengers to allow plenty of extra travel time. The wait for buses dropped to 30 minutes, after a large number of services across the city experienced delays of up to 70 minutes earlier in the day.

Trains also felt the impact of the bad weather; Town Hall station experienced access issues after a partial roof collapse, and Lewisham station remains flooded.

A Traffic Management Centre spokesman said the T1, T2, T3 and T8 airport lines were still affected by the weather and experiencing delays.

Those flying domestically or internationally can expect delays and cancellations.

Sydney Airport's domestic and international terminals were reduced to single runway operations at 7am due to the storm cells and lightning in the area.

It returned to full service later in the morning, but a spokeswoman for the airport said the second runway was opening and closing intermittently because of storm cells.

The storm also resulted in power outages to about 8100 customers in the Sydney CBD, northern Sydney and the Central Coast areas, Ausgrid said. As at 4pm, 1700 customers were still without power.

In Chatswood, a tree brought down powerlines over a road and a tree on powerlines has also affected power supply in Roseville.

Earlier power outages in East Ryde, North Ryde and Marsfield had been fixed, Ausgrid said.

"Given the huge amounts of rain we have already seen today the ground in many places is saturated," Ausgrid chief operating officer Trevor Armstrong said.

"If you add strong winds to that, it can make trees unstable and one of the biggest problems we’ve seen today is trees coming down."

The driver of the big wet is a large low-pressure system over the state which crossed the coast between Sydney and Wollongong.

The rain is expected to hang around at least until mid-afternoon, possibly into the evening commute.

A flood watch is in place for minor to moderate flooding in the river catchments around Sydney, such as the Cooks and Georges rivers, should the intense falls happen over those areas.

Assistant Commissioner Corboy said roads will be "slippery and unpredictable". He urged drivers to reduce their speed and ensure they have enough braking distance for themselves and the vehicle in front.

He also reminded drivers to drop their speed to 40km/h in the likely event that they see emergency vehicles stopped with red and blue lights flashing.

The weather "will be particularly dangerous for motorcycle riders and cyclists," Assistant Commissioner Corboy added.

"If they are on the road, they should wear bright coloured clothing, be seen by other vehicles and share roads safely," he said.

It's not just the intense rain that's expected to cause problems. Damaging winds are also forecast to pick up along the coast in the afternoon as the low moves offshore, bringing with it hazardous surf conditions.

School pick-up time is also looking potentially tricky as the heavy rain overlaps with the strong winds, Bureau of Meteorology spokesman Simon Louis said.

As the rain clears into the evening, it's the strong wind and big surf – in excess of four metres –that he expects will hang around.

Initially Wollongong and the Sydney region will see the hazardous surf, before it extends northwards through Wednesday night and Thursday, as far as the Hunter and mid-north coast.

"Primarily we’re concerned about the hazardous conditions for marine users, so rock fishers, swimmers and small boat users … people need to follow advice from surf lifesaving and water police," Mr Louis said.

The bureau is forecasting 70-100 millimetres to fall on Sydney's Observatory Hill during Wednesday, with Campbelltown looking at 100-150 millimetres and Penrith 70-100 millimetres, according to predictions as of Tuesday afternoon.

The last time Sydney collected more than 100 millimetres in a 24-hour period to 9am was on April 22, 2015, according to bureau data – meaning Wednesday's dump may be the biggest in three years.

Sydney's rainfall so far this month is just over 40.2 millimetres, with the November long-term average sitting on 83.6 millimetres.

So is it enough to break the drought? Unfortunately not.

But while the heavier falls are mostly on the coast, parts of western NSW will still get some "pretty useful totals," Mr Louis said.

The central ranges, central-west slopes and south-west slopes are looking at picking up 20-50mm of rain. "That will certainly be welcome in those regions," Mr Louis said.

"But it's not drought-breaking rain."

with Jamie Berry and Illawarra Mercury