Updated

Illawarra and South Coast residents have been warned to expect heavy rain and damaging winds on Tuesday night with the risk of flash flooding increasing.

A vigorous coastal low pressure system has been developing off the coast of NSW and the Bureau of Meteorology issued a severe weather warning early on Tuesday.

Residents in the Illawarra, South Coast, Central Tablelands, Mid North Coast, Metropolitan, Northern Tablelands, Northern Rivers, Hunter, and North West Slopes and Plains Forecast Districts have been advised to monitor conditions and be prepared for locally destructive winds and heavy rainfall.

Peak gusts in excess of 125km/h are possible for the area from Bondi to Wollongong, which will then extend south to Ulladulla overnight.

Isolated heavy rain has also inundated in Ulladulla and surrounding areas.

The SES issued a statement to advise people in the coastal area from Jervis Bay to Batemans Bay to stay indoors with the same warning to coastal residents from Bulli to Collaroy.

Flash flooding is a high risk throughout the east coast where heavy rain could cut roads without warning. If you do need to be on the roads, drive with caution as we are expecting to see flash flooding by the afternoon commute.

NSW SES Deputy Commissioner Debbie Platz reminded residents that flash flooding is dangerous and unpredictable.

"This particular low is now going to start to impact coastal areas of NSW from tonight, from the areas between Coffs Harbour in the north and Bega in the south," she said on Tuesday.

"It is likely that this system will bring significant weather to these coastal fringe areas.

"This weather will include damaging surf, coastal erosion, significant moderate to heavy rainfall and thunderstorms along with damaging winds.

"This is a very dynamic and fast-moving system. Very different to the systems we have seen in recent times. We do expect the rainfall and winds will ease from Thursday."

Kiama Blowhole on Tuesday. Photo: Matty Taylor

Plan your journey ahead where possible. Check Live Traffic NSW for updates on closures or hazards on your route and the Hazards Near Me app.

Six-hourly rainfall totals between 70-90mm are possible, with isolated totals up to 120mm. A separate Coastal Hazard Warning is also current for parts of the NSW coast.

Locations which may be affected include Taree, Newcastle, Gosford, Sydney, Wollongong, Armidale, Nowra and Batemans Bay.

The State Emergency Service advises that people should:

* Move vehicles under cover or away from trees.

* Secure or put away loose items around your house, yard and balcony.

* Keep at least 8 metres away from fallen power lines or objects that may be energised, such as fences.

* Trees that have been damaged by fire are likely to be more unstable and more likely to fall.

* Report fallen power lines to either Ausgrid (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093) as shown on your power bill.

* Don't drive, ride or walk through flood water.

* Keep clear of creeks and storm drains.

* If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue.

* Be aware that run-off from rainfall in fire affected areas may behave differently and be more rapid. It may also contain debris such as ash, soil, trees and rocks.

* After bushfires, heavy rain and the loss of foliage can make the ground soft and heavy, leading to a greater chance of landslides.

* Stay vigilant and monitor conditions. Note that the landscape may have changed following bushfires.

* For emergency help in floods and storms, ring your local SES Unit on 132 500.

To keep up to date with the weather conditions, visit the Bureau of Meteorology website or call 1300 659 210.