Warning for Sydney residents as rain bomb moves south: 'Be prepared'

• Sydney bracing for dangerous weather

• Intense rainfall, dangerous winds and flash flooding

• People in Sydney and south coast need to be ready to evacuate

Sydneysiders should brace for life-threatening conditions in the coming hours, as the weather system lashing the north coast of NSW heads south.

On Tuesday, the Bureau of Meteorology issued a severe weather warning for heavy rainfall and damaging winds for Sydney, parts of the Hunter, the South Coast and parts of Central and Southern Tablelands, as the deepening low off the coast in northern NSW drifts south.

"At this stage, the system is forecast to approach central and southeastern districts tonight or during Wednesday," the Bureau said on Tuesday.

"Uncertainty exists on its exact timing and where the most significant impacts will occur."

It warned intense, six-hourly rainfall totals of up to 200mm are likely to hit with thunderstorms south of Gosford.

"Life-threatening flash flooding is possible," the bureau warned.

NSW Premier Dominic Perrottet advised people in Sydney and further south to be ready.

"We know that at the moment this is focused on the north," he said on Tuesday.

"But very quickly as we're seeing in metropolitan Sydney now, severe and heavy rainfall and then it will move to the south.

"So, to all people, particularly across the east coast of our state, these issues may come your way."

BOM Meteorologist Dean Narramore said the rain in Sydney is going to stretch across the whole Metropolitan region, right into the western parts of the area.

There could be moderate flooding in communities in the Hawkesbury and Nepean areas.

Now is the time to prepare for Sydney storms

On Tuesday morning, Minister for Emergency Services and Resilience, Steph Cooke, said now is the time to prepare for people in Sydney and south coast.

"As this event moves south, people in Sydney and on the south coast have time," she said.

"We have seen what unfolded in the northern parts of the state over the past few days. The time to prepare is now."

Ms Cooke said people who are subject to an evacuation order should heed that advice.

She reiterated the best way to help emergency services is to be prepared.

How to prepare for severe storms

"Have an emergency services kit prepared with your medications, with your important documents, your lap top computers, mobile phones and chargers, so we can as best as possible keep track of where you are, that you can contact us if you need to," she said.

"But more importantly, that you can be prepared to evacuate should the need arise."

This week, people were meant to return to offices in Sydney's CBD, however, Mr Perrottet stressed people should be cautious.

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"Based on, obviously, the advice from the bureau, it's going to be a difficult week in our city a as we've seen those heavy rains go from the north," he said.

"We're seeing it in Sydney today and we'd expect that to go further south towards the back end of the week.

"Ultimately, right now, we say to people across our city and across our state, look after yourself, follow the advice, stay safe and we'll all get through."

Loss of life in flooded towns likely

Towns across northern NSW, including Lismore and Ballina are underwater and rivers are peaking even as the rain stopped.

Ms Cooke urged people to prepare for loss of life.

"I think that it is unrealistic that a disaster of this magnitude will mean that there are no lives lost," she told reporters.

Hundreds of people on the north coast are still unaccounted for and one person is feared dead.

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