THE Hunter is among several parts of the state warned to brace for potentially intense rainfall in the early hours of Thursday.

A flood emergency is unfolding in parts of Sydney on Thursday morning, with 30 new evacuation orders issued overnight.

Officials are warning the system is doubling back, putting the Central Coast and Hunter in its expected path.

NSW Premier Dominic Perrottet said that it would likely "get worse before it gets better across our state", flagging the day ahead for Newcastle and the Hunter as a particular concern.

He said more than 500,000 people were subject to weather warnings or evacuation orders.

"We've got through every one of these difficulties of the past, and we will get through this together again," he said.

The Bureau of Meteorology said an east coast low was at 6am about 100 kilometres east of Newcastle, and it was "expected to deepen and continue moving towards the Hunter or southern Mid North Coast districts" in the next few hours.

The system is expected to weaken later tonight.

Warnings include heavy rainfall, with six-hourly totals of between 80mm and 120mm "likely".

"Locally intense rainfall leading to dangerous and life-threatening flash flooding is possible with thunderstorms with six-hourly rainfall totals in excess of 200 mm possible," the Bureau warned.

Wind gusts in excess of 90km/h may also develop along the coastal fringe.

Parts of Sydney have been buffeted overnight, joining Lismore and the north coast in evacuating their homes.

Rain totals on the Central Coast since 9am on Wednesday have been significantly higher than in the region, bureau data shows, with 109.4mm at Mangrove Mountain, 71.4mm at Cooranbong and 69.2 at Gosford.

86mm of rainfall was recorded in the six hours to 3:45pm at Warragamba, with another 73mm recorded in the following period to 9:45pm

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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.

Newcastle is at risk of flash flooding as the East Coast Low moves down the coast overnight into Thursday.

Six-hourly rainfall totals between 80 and 120mm of rain is possible.

There is no forecast time for when the worst of the low will hit as it has moved slower than expected, but the Bureau of Meteorology has warned of high winds, rough surf and abnormally high tides.

A severe weather warning extends from north of Newcastle to south of Wollongong, carrying with it heavy to intense rainfall and damaging winds which can reach up to 90km/h

Heavy to torrential rain is likely to the south of the low, affecting the Hunter, Central Coast, Sydney, Illawarra and the South Coast.

A hazardous surf warning is in place for the entire NSW coast, causing abnormally high tides.

A high pressure system near New Zealand directed the strong easterly flow over NSW this week.

East Coast Lows can be slow-moving, and as a result the effects usually last longer than normal low pressure systems or cold fronts.

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Newcastle City Police reported an increase in motor vehicle collisions because of the wet weather, reminding drivers to slow down, turn on headlights and leave a large gap between vehicles in front.

SES and emergency services are pleading with drivers and the community in general to avoid unnecessary travel, and not enter any flood waters that may have formed under any circumstances.

Sydney trains and NSW TrainLink networks have been impacted, with some running to an altered frequency or stopping pattern.

Transport for NSW chief operations officer Howard Collins advised the community to expect delays to continue this week as widespread heavy rainfall and flood risks continue along the east coast.

"Some NSW Trainlink regional coach and train services may not operate as timetabled. While delays and cancellations cannot be avoided at this time, every effort is being made to provide customers with safe alternatives," he said.

For SES support call 132 500 or in a life threatening emergency call 000.

* NSW SES: www.ses.nsw.gov.au

*Live Traffic: www.livetraffic.com

* Latest NSW Warnings: www.bom.gov.au/nsw/warnings/

* Rainfall Forecasts: www.bom.gov.au/australia/meteye/

* BOM NSW Twitter: www.twitter.com/BOM_NSW

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