At least five million people are being forced to take shelter from a “bomb cyclone” that has unleashed torrential rain, savage winds and waves of up to eight metres on the NSW coast, with the blitz of bad weather expected to intensify into Wednesday.

Residents along the coast of Sydney, Jervis Bay, Ulladulla and Batemans Bay were urged to stay inside on Tuesday as the complex low-pressure system lashed the state with winds reaching 110 kilometres per hour, and downpours in some areas that dumped a month’s worth of rain in just a few hours.

Wet and stormy weather is starting to impact the NSW coast.

“It’s a very significant, widespread, dynamic and complex system,” warned SES deputy commissioner Debbie Platz.

“We do expect that we will see wind gusts in pockets of up to 125 kilometres per hour. We do expect that we will see rainfall from moderate to heavy in many areas, from 50 millimetres right through to 150 millimetres, and in some pockets isolated rainfalls in excess of 200 millimetres.”

Residents of a handful of beachfront homes at The Entrance and Wamberal Beach on the Central Coast were ordered to evacuate on Tuesday afternoon as wild surf ate away at the coast.

Emergency Services Minister Jihad Dib said the storm system spanned from the Mid-North Coast to Bega, although communities such as Taree in the state’s north, still reeling from May’s flood disaster, were spared the worst of the heavy rain.

Huge surf battered the NSW coast on Tuesday.

“Just the sheer size of it is quite enormous,” Dib said, warning that it was unlike the other recent storm systems that had battered NSW. “This one will be a much more intense and a much sharper system.

“The terrible thing is that the situation is going to worsen over the course of the next 24 hours, particularly later this afternoon and into the evening tomorrow. So once again, we’re asking people to be as prepared as they possibly can be.”

Authorities urged people to move their cars away from large trees as heavy rain hit saturated ground and fierce winds began to tear down branches and powerlines, cutting power to 21,000 people across the Hunter, Central Coast and Sydney.

Although much of the coast was on alert for minor to moderate riverine flooding, flash flooding unleashed by possible downpours of 200 millimetres posed the greatest threat, particularly between Newcastle and the Illawarra.

Fishers brave dangerous conditions in Newcastle as the fierce coastal low sets in.

The South Coast will encounter the greatest risk from flash flooding as the weather system continues its poleward crawl before spinning into the Tasman late in the week.

What is a ‘bomb cyclone’?

Meteorological jargon used to describe the unusual low-pressure storm system has included “bomb cyclone”, “bombogenesis” and “explosive cyclogenesis”.

These terms all refer to the rapid intensification of the weather system. A dramatic plunge in pressure – about 20 to 30 hectopascals over the past day or two – transformed the low into a severe storm system. Regions of low pressure in the atmosphere funnel air upwards, which triggers the development of storm clouds and powerful winds.

The system the bureau called a “vigorous coastal low” spiralled down the coast while generating storm-force offshore winds comparable to a category two cyclone, adjunct professor of environmental geography at CQUniversity Steve Turton said.

“That’s certainly going to increase the wave energy and the swells and so on coming onto the coast. We’re looking at over 5 million people likely to be affected by this system,” he said.

Abnormally warm water off the NSW coast helped fuel the rapid intensification, Turton said. Warmer oceans turbocharge storm energy and supply weather systems with moisture for heavy rain.

A region of ocean water brewing 1 to 3 degrees above the long-term average also contributed to Tropical Cyclone Alfred and May’s flood disaster, Turton said.

“That pool of warm water off the coast will certainly be providing additional energy into this system and increasing the risk for more rapid intensification and flash flooding,” he said.

About 90 per cent of the Earth’s extra heat caused by the burning of fossil fuels is trapped within the ocean, he noted.

NSW east coast weather warnings

A severe weather warning for damaging, locally destructive winds remained in place for Sydney, the Illawarra, and parts of Mid North Coast, Hunter, South Coast, Central Tablelands, Southern Tablelands, Northern Tablelands and Northern Rivers as of 4pm Tuesday.

Rock fishers, boaters and swimmers were urged to stay away from the ocean under a hazardous surf warning in place from the Hunter down to Eden. Coastal erosion and damage to infrastructure may strike the coastline between Seal Rocks and the NSW-Victorian border.

Southern parts of the Mid North Coast, the Hunter, Sydney-Illawarra Coast and Snowy catchments were under flood watch for minor flooding, while moderate flooding was possible for the Hawkesbury-Neapean.

Public transport warnings and flight cancellations

Transport for NSW operations management executive director Craig Moran advised people to avoid non-essential travel, and if they did need to travel, to give themselves extra time and be prepared for disruptions.

“There may be sudden road closures or there may be sudden delays on the public transport system. Everything is holding up OK at the moment, but … just be prepared that circumstances can change,” he said shortly after midday on Tuesday.

Delayed aircraft line up to take off at Sydney Airport on Tuesday.

A number of low-lying roads were closed in the Shoalhaven area on the state’s South Coast, which had received high amounts of rain overnight, he said.

Sydney Airport switched to using just its east-west runway on Tuesday afternoon due to strong south-westerly winds, leaving two of its three runways closed.

The agency said delays are expected, with passengers advised to check with their airlines regarding any changes to travel arrangements.

Qantas and Virgin cancelled dozens of flights in and out of Sydney on Tuesday.

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