Sydney and Brisbane are being warned to brace for torrential rain and widespread flash flooding as ex-tropical Cyclone Debbie moves down the east coast of Australia.

Queenslanders are being told to go home at lunchtime today, as the state prepares to be drenched with a month's worth of rain over the next 24 hours, according to Daily Mail.

In NSW, residents will be lashed by heavy rain this afternoon, with flash flooding in the state's northeast and damaging coastal winds north of Sydney.

Melbourne is on track for four consecutive days beneath 20 degrees, with freezing temperatures through the nights and the chance of snow in Victoria's Alpine region.

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The gloomy weather should mark Sydney's wettest March in a staggering 42 years.

Queenslanders have been warned to prepare for the worst with flash flooding now a serious concern across the state.

"As the low tracks down the coast towards Brisbane we can expect falls of up to 200mm or more in southeast Queensland," a BoM spokesman said.

A car park in Gold Coast has already been deluged by the heavy rains, while the Gold Coast TItans NRL team were forced to cancel training after their field flooded.

The Gold Coast Titans NRL team were forced to cancel training after their field was flooded. Photo/Gold Coast Titans/Twitter
The Gold Coast Titans NRL team were forced to cancel training after their field was flooded. Photo/Gold Coast Titans/Twitter

A severe weather warning remains in place for the New South Wales coastline from Lismore to Sydney as a cold front approaches from the southwest.

Residents in Lismore, Grafton, Coffs Harbour Glen Innes and Inverell will be dumped with most of the rain while Sydney, Gosford, Newcastle and Port Macquarie will be hit by damaging winds.

The Northern Rivers region can expect 100mm over a 24-hour period.

Flooding in Mackay's CBD after tropical cyclone Debbie hit the north Queensland region. Photo/AAP
Flooding in Mackay's CBD after tropical cyclone Debbie hit the north Queensland region. Photo/AAP
A car - thankfully with no one inside - sits in floodwater outside a Gold Coast hospital today. Photo/AAP
A car - thankfully with no one inside - sits in floodwater outside a Gold Coast hospital today. Photo/AAP

Sydneysiders can expect showers to start this afternoon and continue into the evening with a maximum of 45 millimetres expected.

"It is likely that some locations will exceed more than 200mm," the bureau said on Wednesday.

The showers will linger on tomorrow for Sydney but it won't be as much as the previous day's dumping, with about 6mm forecast.