An enormous clean-up awaits as a rare and damaging storm hits Australia’s east coast, with residents ordered to evacuate amid threats of flash flooding, damaging wind and coastal erosion.

Areas stretching from Queensland‘s Lockyer Valley to Bega on NSW’s south coast are being warned of severe weather from a “vigorous” coastal low as the system lingers offshore.

The storm was expected to track south before turning back out into the Tasman Sea during Wednesday.

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Wind gusts in the NSW Illawarra region were expected to exceed 125km/h, with isolated rainfall up to 120mm in six hours threatening to deliver flash flooding in Wollongong and surrounding areas.

The NSW SES late on Tuesday said 23 active warnings remained across the state.

More than 1300 incidents had been reported, including two flood rescues, it said, as conditions were expected to worsen overnight.

More than 29,400 homes are waking up without power on Wednesday morning.

Ausgrid listed 10,391 customers experiencing power outages across their services in Sydney, Newcastle and the Central Coast region.

At least 17,077 Endeavour customers in Western Sydney, Blue Mountains and South Coast region are also without power, while a further 2150 residents were without power in the remainder of the state through Essential Energy.

Across the road from where residents were being warned to evacuate amid coastal erosion on the Central Coast, Dunleith Tourist Park manager Ryan Lloyd told AAP beaches were being heavily affected as wind gusts and swells intensified.

“It’s pretty wild - pretty full on - a bit scary for the homeowners too,” he said.

There were patches of blue in the sky, but they were otherwise surrounded by dark clouds.

“It’s quite ominous, quite eerie.”

But the rain was less of a concern than the strong winds.

“We’ve already had plenty of tree branches down ... they’re dropping limbs everywhere,” Lloyd said.

“It’s just battening down now, just wait for it to blow over - pardon the pun - then a massive clean-up for the next couple of days.”

The Bureau of Meteorology was predicting another low-pressure system to enter the storm’s path later on Wednesday.

“We see another low-pressure system, really dumb-belling around that first one, that will really intensify and reinforce some of those winds and rain across the south coast,” senior meteorologist Jonathan How said.

Weather phenomena expert Andrew Dowdy said the storm was likely to have severe impacts.

The system extended about five kilometres into the sky, with two low pressure systems interacting with one another in a phenomenon known to meteorologists as the Fujiwhara effect.

“It is very rare to see two east coast lows spiralling around each other like this,” the University of Melbourne associate professor said.

Multiple areas south of Wollongong received more than 50mm of rain on Tuesday as the storm intensified, with Ulladulla hit with almost 100mm in the nine hours to 6pm.

There is a chance Warragamba Dam west of Sydney could spill as a result of the storm, threatening flooding in the Hawkesbury-Nepean catchment it flows into.

Any spill would depend on the actual rainfall and may not occur until days after the storm’s expected end, a WaterNSW spokesman said.

Dozens of flights were cancelled at Sydney Airport on Tuesday, while hundreds were delayed as wild weather forced the closure of all but one runway.

The airport still warning customers on Wednesday morning to monitor updates from your airline regarding the status of your flight.

Significant rainfall observations:

80.5 mm was recorded at Oranmeir in the 6 hours to 2.05am.

108 mm was recorded at Nowra in the 6 hours to 12.02am.

133 mm was recorded at Morton in the 6 hours to 11.43pm.

116 mm was recorded at Vincentia in the 6 hours to 9.37pm.

121 mm was recorded at Greenwell Point in the 6 hours to 9.10pm.

112 mm was recorded at Currarong in the 6 hours to 8.42pm.

Significant wind observations:

100 km/h gust was recorded at Jervis Bay Airfield at 7.36pm.

102 km/h gust was recorded at Point Perpendicular at 7.33pm.

104 km/h gust was recorded at Ulladulla at 7.2 pm.

102 km/h gust was recorded at Penrith at 6.27pm.

91 km/h gust was recorded at Wollongong at 6.21pm.

91 km/h gust was recorded at Kiama at 4.04pm.