Millions of residents from Coffs Harbour south to Bega are in the path of a fast-moving, severe low pressure system intensifying off the NSW north coast.
The rapidly deepening system is known as a "bomb cyclone", although the meteorological term is generally used sparingly so as not to incite panic, the Bureau of Meteorology said.
It's already sparked heavy rain, strong winds and hazardous surf and up to 120mm of rain is expected to lash Greater Sydney alone over the next six hours.
NSW SES state operations manager Dallas Burnes said the emergency calls have already started to roll across the state as the system intensifies.
"We're already starting to see the impacts - 600 incidents responded to in the last 24 hours and over a thousand of our volunteers out helping their communities already," he told Nine's Today program on Tuesday.
"We are really concerned about flash flooding, particularly around the commute home for people this afternoon, as we see those rainfall figures escalate throughout the day," Mr Burnes warned.
The peak impact of the system is forecast to happen on Wednesday, and there's a risk of flash flooding at Wallis Lake near Taree on the mid north coast of NSW.
Taree was one of the towns hard hit by floods in May that killed five people and damaged thousands of properties.
Weather warnings have been issued for Taree, Newcastle, Gosford, Sydney, Wollongong and Port Macquarie
The mid north coast region is an area of concern for emergency services because of the soil is still saturated from the May floods.
"If they get those gusts up to 125km an hour, we are going to see trees come down," Mr Burnes added.
The deepening coastal low will also result in dangerous beach conditions, coastal erosion and damage to the NSW coast from Seal Bay to Batemans Bay, the bureau warned.
The heaviest rain is expected on the state's central coast, with totals of up to 200mm possible, although a "subtle shift" in conditions could move those falls to Sydney or the mid-north coast.
Coastal communities were being urged to prepare ahead of the storm by tying down loose items and moving cars away from trees.
Conditions are expected to ease by Thursday.
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