About 30,000 customers were without power on Wednesday morning as a damaging storm smashes Australia’s east coast.

The ominous “bomb cyclone” has unleashed destructive winds, and lashed towns on the central and south NSW coasts with intense rainfall.

Alerts on Wednesday morning were issued for residents to stay indoors in Jervis Bay, Ulladulla and Batemans Bay, on the south coast, due to heavy rainfall and damaging and destructive winds.

Others at the coastal hamlet of Burrill Lake were urged to evacuate due to “dangerous flooding”.

People in parts of Sanctuary Point, on the south coast, were told to “immediately” head to the higher levels of a sturdy multistorey building after it became too late to evacuate.

“You may now be trapped without power, water, and other essential services and it may be dangerous for NSW SES to rescue you.”

Wild seas pounding coastal shores had on Tuesday afternoon forced evacuations at Wamberal Beach, on the central coast, as homes teetered on eroded sand dunes.

Meanwhile, the Bureau of Meteorology predicts another low-pressure system will 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 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.

The NSW SES had responded to more than 1300 incidents by Tuesday afternoon, including two flood rescues.

The system had also hit transport, with commuters urged to avoid all non-essential travel across the rail network on Wednesday after damage to infrastructure.

Severe weather warnings extended from Queensland’s Lockyer Valley to Bega on NSW’s south coast.

The “vigorous” coastal low has lingered offshore and was expected to track south before turning back out into the Tasman Sea on Wednesday.

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

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

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

Any spill would depend on actual rainfall and may not come 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.

Across the road from where residents were being warned to evacuate amid coastal erosion on the central coast, Dunleith Tourist Park manager Ryan Lloyd said beaches were being pounded as wind gusts and swells intensified.

“It’s pretty wild — pretty full on — a bit scary for the home-owners 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.”

–with AAP