Millions of residents are in the path of a “cyclogenesis” low-pressure weather system hitting coastal communities of NSW from Tuesday morning.

Hundreds of emergency services personnel, helicopters and specialist vehicles are on standby as the wild storm tracks towards regions still recovering from deadly floods.

The so-called “bomb cyclone” — a reference to the explosive nature of its development — was “rapidly deepening” on Tuesday morning amid fears it could be even more dramatic than originally thought.

The weather bureau said it would strengthen on Tuesday, and issued multiple warnings for heavy rain, strong winds and hazardous surf for a vast stretch of the NSW coast.

A severe weather warning on Tuesday morning said a “vigorous” coastal low would develop off the NSW mid-north coast early Tuesday.

It would slowly track south and approach the central NSW coast later on Tuesday before turning and moving offshore into the Tasman Sea on Wednesday, the Bureau of Meteorology said.

The wild weather, with wind gusts of up to 125km/h, could hit early Tuesday along the Hunter and mid-north coast before slamming Sydney, Wollongong and the central coast from later in the day.

“Six-hourly rainfall totals between 70 to 90 millimetres are possible, with isolated totals up to 120 millimetres,” the BOM said.

The peak effects would be felt on Wednesday from Coffs Harbour south to Bega. It includes the risk of flash-flooding at Wallis Lake near Taree, one of the towns hard hit by floods in May that killed five people and damaged thousands of properties.

The NSW State Emergency Service has about 400 personnel ready to be deployed.

Bureau meteorologist Angus Hines said the system met the definition of a “cyclogenesis”, the formation of a low-pressure area.

Weatherzone said the “explosive process” of the system’s formation off NSW would see its central pressure “drop rapidly”.

“The [rapid drop] classifies the system as a bombing low.”

NSW SES deputy commissioner Debbie Platz said authorities had been monitoring the “very dynamic and fast-moving system” system since last week.

“It’s likely that this system will bring significant weather to these coastal fringe areas,” Platz said.

“We do expect that as a result of that we will have flash flooding, as opposed to riverine flooding, that is not to discount riverine flooding.”

Bureau hazard preparedness manager Steve Bernasconi said the system would be at its most intense on Wednesday and produce destructive winds and coastal erosion to large stretches of coast.

Damaging winds are expected in Sydney, the Hunter Valley and Illawarra regions.

Hazardous surf is expected along the coast, the bureau cautioned.

The heaviest rain is expected over the NSW central coast, with totals of up to 200 millimetres possible, although a “subtle shift” in conditions could move those falls to Sydney or the mid-north coast.

Coastal communities are being urged to prepare before the impact of the storm by tying down loose items and moving cars away from trees.

“As we move into Thursday, rain will ease, the winds and the surf may still remain a hazard, and on Friday conditions are expected to improve,” Bernasconi said.

-with AAP