Sydney, Brisbane, Melbourne weather: Mega rain bomb detonates over east coast bringing flash floods and severe weather warnings, as 'five-in-one' lightning strike hits the city

A month's worth of rain smashed large parts of Australia's east coast overnight, triggering flash floods and evacuation warnings in Sydney's southwest.

The rain bomb exploded over much of NSW on Thursday night, with over 250mm of rain falling in parts of the Illawarra and the state's south-east.

In Sydney, where almost 60mm of rain fell in the last 24 hours, an incredible 'five-in-one' lightning strike hit shortly after 8pm on Thursday, with two bolts appearing to strike the Harbour Bridge.

The NSW SES has received over 450 calls since 6pm last night, with residents in Picton and Menangle, in Sydney's south-west, being told to prepare for possible evacuations.

Emergency workers will be conducting door knocks in affected areas - including Windsor, North Richmond, the Hawkesbury, and the Nepean - as the Nepean River is expected to swell beyond it's banks on Friday morning.

People in Shoalhaven Heads, on the state's south coast, are also being told to prepare for possible evacuation as over 60 rescues have been completed across the region.

WaterNSW also confirmed Warragamba Dam has begun spilling as of 4.20am Friday, at a rate of 150 gigalitres per day.

The overspill will flow into the Nepean River, which could affect those living in low-lying areas along its banks in Sydney's south-west. 

Authorities are advising people to keep a close eye on the rapidly changing situation.

With rain forecast well into the long weekend, it may be a while before NSW - as well as Perth and some parts of Victoria - get any respite from the wet weather.

BOM meteorologist Angus Hines said significant weather will impact both the east and west coasts of Australia as we finish off the first week of winter, with some areas to be hit with a 250mm downpour.

'The action happening in terms of rainfall is happening in New South Wales, as well as far south west of WA,' he said. 

'The wet weather will kick off in both of those regions late on Thursday and it is expected to be quite wet on both edges of the country through Friday.'

By Friday, some areas in northeastern Victoria will also be experiencing heavy rainfall.

Mr Hines said more than 100mm of rainfall, if not more, was predicted to hit the Illawarra region and the south coast across the three days.

'A surface trough extending from the Illawarra region into the Tasman Sea will deepen during Thursday in response to an upper low over New South Wales,' the BOM website states. 

'Rainfall totals between 100 and 150mm are likely with isolated totals of 250mm possible.'   

A flood watch warning has been issued for parts of Wollongong, Nowra, Bowral, Ulladulla, Taralga and Nerriga. 

Rivers predicted for minor flooding include the Cooks River, Shoalhaven River, St Georges Basin, the Moruya and Deua Rivers.

Particular rivers on flood watch are the Upper Nepean River, Hawkesbury and Lower Nepean Rivers, Colo River, Georges and Woronora Rinvers for moderate flooding.

Along with rain, the Bureau has outlined cooler temperatures across most of the states.

But a reprieve from heavy rain is in sight.

'(But) on Friday, initially the low moves towards the coast but then it does a U-turn in the morning and starts to move away from the country, meaning in the second half of the day on Friday that rain will really begin to clear out with brighter weather,' Mr Hines said.

Severe weather warnings remain active between Oberon in the central tablelands and the southern coast town of Uladulla.

The bureau predicts parts of the Illawarra district, areas of the south coast and southern sections of Sydney could cop more than 100mm of rain between Thursday and Sunday.

The alert came after the bureau in May warned Australia could be hit with the return of a La Nina weather pattern, which typically brings wetter-than-usual conditions to the nation's east.

There was a 50-50 chance the weather system might form in the Pacific Ocean later in 2024, it said at the time.

In April, the bureau declared an end to an El Nino weather event, which generally brings hotter, drier weather to the country's east.