Sydney, Brisbane, Melbourne weather: When will it end - urgent warnings about 'dangerous' floods as severe weather causes chaos on Australia's east coast

All residents of NSW have been told not to leave their homes unless their trips are 'necessary', as extreme weather wreaks havoc across the east coast.

Trains, roads and Sydney Airport are in chaos as authorities struggle to cope with what is being called a 'rain bomb.

Two major weather systems have merged together, bringing as much as 300mm of rain from south-east Queensland to NSW's South Coast on Friday. 

The rare weather phenomenon, known as a Black Nor'easter, plunged some areas into darkness in the middle of the day.

The Bureau of Meteorology (BoM) said large parts of the NSW east coast will get extreme rainfall and are at risk of flash flooding throughout Friday evening.

'Our severe weather warning at the moment is covering a large area of eastern and southern parts of NSW, stretching from around Newcastle in the Hunter area down to Bega on the South Coast,' Senior Meteorologist Angus Hines said.

That includes 'all of the greater Sydney area, the Illawarra area and pushing into the Central Tablelands'.

In an update at 5pm, BoM said 'A trough is forecast to deepen over eastern NSW and along central parts of the coast today and Saturday, under the influence of a slow-moving upper-level low over central NSW. 

'Areas of heavy rain and gusty showers and a few thunderstorms will become more widespread south of about Newcastle this evening, including over the ranges and tablelands. 

'Severe weather is expected to gradually shift south overnight and Saturday, while easing from the north as the trough moves southeast to the Tasman Sea.'

NSW SES Commissioner Carleen York said the wild weather hammering down across the state made driving dangerous and advised residents to stay indoors.

'I'm asking people that If it's not a necessary trip to put it off to another day,' she said.

More than 550 people have called the SES for help in the past 24 hours, with seven flood rescues conducted since Monday.

In a post on X at 4pm, the NSW SES said 'Low lying areas of eastern parts of Chipping Norton (are) flooding - Prepare to evacuate. Watch and Act.'

Chipping Norton is a south-western Sydney suburb, 27kilometres from the CBD. 

The rain is so severe that Warragamba Dam, which supplies most of Sydney's water, is expected to spill over on Monday morning.

Thought the wet weather is set to ease up by Sunday, the dam is already at 96 per cent capacity.

It needs just 90mm of rain to spill, with weather forecasters predicting 150mm is on the way.

'What's important about that is that the spill will occur likely when the rainfall event has moved on, so it is very important that the community remain vigilant,' Water NSW chief executive Andrew George told reporters on Friday.