Millions of Aussies set to be drenched in 'multi-state thunderstorm outbreak'

After days of wet weather and thunderstorms across NSW and the ACT, forecasters are warning the worst is yet to come.

According to the Bureau of Meteorology [BOM], Thursday will see the "highest impact" of rain with thunderstorms to sweep across large parts of the states.

BOM forecaster David Wilke said there is a high pressure storm sitting near Tasmania, which is directing cooler winds along the east coast.

"We've seen pretty cloudy conditions and some showers throughout parts of the east, particularly the northeast over the past couple of days," Mr Wilke said.

"But over the next couple of days, starting Wednesday we're going to see a cold front moving through the state."

He said as the storm moves through NSW, residents can expect some widespread showers and storms, with the southern part of the state potentially getting some "reasonable" rain.

However, BOM meteorologist, Jonathan How said the wet weather could impact millions of Australians.

On Thursday, there is a chance of heavy rain in Tasmania, but then the focus will shift to eastern NSW and southeast Queensland, with the chance of a "multi-state thunderstorm outbreak".

"Severe thunderstorms are likely to have the potential to cause damage to property, flash flooding and dangerous conditions," Mr How said.

"Sydney, Brisbane and Canberra could all be impacted."

Thunderstorms expected across the east

Mr Wilke said it will take a few days for the storm to move through, with inland parts of NSW getting a little activity on Wednesday, while more eastern parts will cop it on Thursday and the dreary weather lingering until Friday.

"The most dynamic day at this stage does look to be Thursday," he said.

"That day looks the most likely to see severe thunderstorms, though there is also the chance through the inland on the Wednesday as well."

Much of the eastern part of NSW could see severe thunderstorms on Thursday, extending from Canberra to Sydney and then up through the Great Dividing Range.

"So, really quite a broad area we've got the risk of thunderstorms becoming severe, and so at this stage we're monitoring for the possibility of large hail, damaging wind gusts and heavy rain," Mr Wilke said.

"It could be any three of those associated with severe thunderstorm, so it does look like quite a significant day."

The Bureau warned flash flooding could be an issue across NSW and the ACT.

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Most regions will see some rainfall as the high pressure system moves across NSW

Rainfall heaviest on Wednesday night for Melbourne

The Bureau of Meteorology also forecasts thunderstorms along the east of Victoria in the early morning and isolated showers elsewhere, which may become thunderstorms throughout the late morning.

However, Melbourne is expected to get the most rainfall on Wednesday night.

On Thursday, a cold front will move quickly across southern and central parts of Queensland.

Though the threat of severe thunderstorms is only expected in the southeast, there is a medium chance of showers and thunderstorms in the eastern districts and a slight chance of a thunderstorm or shower in the northwest.

"Risk of supercell development also with locally destructive winds, giant hail, intense rainfall possible," the Bureau said of Queensland's southeast on Thursday.

Sunshine to return by the weekend

Mr Wilke said it would be worth keep an eye on forecasts and warnings over the next few days, to ensure you are prepared.

While it has rained for most of the week, when the vast majority of people in NSW got to enjoy some new freedoms, there does seem to be some sunshine ahead.

Come the weekend most places in NSW will see clear skies as the front moves through, but showers may linger in southern inland parts of the state Mr Wilke said.

"Generally not expecting a whole lot of rain, and any rain we do see will be easing through the Saturday, but most places will see a bit of an easing through the weekend," Mr Wilke said.

"The caveat is that it will be still quite cool for this time of the year, at least we're not expecting a return to heat just yet, so most places are seeing the cool but hopefully clearing weekend."

The Bureau has issued an initial flood warning for the Severn River at Ashford, while minor flooding has already occurred along the Lachlan River at Euabalong.

As for Victoria, BOM has put out a minor flood warning for Gippsland, North East and Central Victoria.

"Minor flooding is possible from Thursday afternoon in the catchments of the flood watch area," the Bureau said.

"A low pressure system and trough will enter western Victoria during late Wednesday afternoon, reaching central districts at night before weakening early on Thursday.

"Catchments in the flood watch area are moderately wet."

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