Australia’s wild and wet summer is about to unleash another drenching, with thunderstorms, heavy rain and floods forecast for NSW and Victoria.

Weatherzone meteorologist Ben Domensino said the soggy spell was due to an unusual weather pattern – with Australia being squeezed by four low-pressure systems.

“This is an unusual synoptic pattern and it’s likely to bring brollies out across Australia in the coming days,” he said.

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It’s likely to be a wet and stormy weekend across much of NSW, with leftover moisture from a tropical cyclone creating unstable conditions.

Thunderstorms have already begun rolling in over the northern ranges, around the Barrington Tops area, with the potential for severe storms over the central inland areas of the state later on Thursday.

“The greatest risk is that we may see damaging winds or heavy rainfall,” the Bureau of Meteorology’s Helen Kirkup said.

Friday would bring more of the same, she said, with the area from Sydney down to the south coast and west to the Riverina in the firing line.

“With the thunderstorms, where they pass we could see some much heavier rainfalls, and some of this may be very intense and could lead to flash flooding.”

“We’re not ruling out the chance of large hail either.”

Lingering moisture from Tropical Cyclone Lucas is combining with front create the system causing the havoc, which is expected to begin clearing out by Sunday.

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Further south, thunderstorms are also tipped to dump more rain on large swathes of Victoria.

Between 40 and 70 millimetres of rain is expected to fall in Victoria’s north-east and central areas on Friday as a low-pressure system moves east.

Isolated totals could top 100 millimetres within the warning area, which spans from Geelong to Wodonga on the NSW border.

The storm front is also forecast to produce damaging wind gusts of 90km/h and could rise above 100km/h in elevated areas.

The Bureau of Meteorology’s Diana Eadie said unseasonably wet conditions in the past week had prompted a warning for minor to moderate flooding in the state’s northeast.

“With this rainfall, we’re more likely to see further riverine rises,” the senior meteorologist said on Thursday.

The damaging winds and heavy rain are expected to extend to Melbourne.

The heaviest falls are forecast for the city’s east and south, while the north will face the worst gusts.

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Victoria State Emergency Service deputy chief officer David Baker said there was potential for flash flooding across Melbourne.

“The simple message is: Do not enter flooded waters,” he said.

“Last Friday alone, our crews had to attend to three rescues when vehicles had entered waters.”

Mr Baker is urging people to start preparing for the storm around their homes, with the soft soil and strong winds increasing the danger of falling trees.

“The next 48 to 72 hours will prove challenging for Victorians,” he said.

In the north, the low – if it develops – is likely to be a tropical low-pressure system somewhere near the NT’s Top End or over the Gulf of Carpentaria. According to Weatherzone, it even has the potential to become a tropical cyclone towards the end of the week.

As it develops, widespread rain and thunderstorms are likely to develop over the Northern Territory and Queensland, with more destructive conditions possible if it becomes a cyclone and approaches land.

The unusual weather pattern will also be felt in the west, where there has already been flooding rain in the parts of Western Australia this week.

Weatherzone said that low was expected to drift south on Friday and Saturday. It could bring heavy rain in western and south-western districts of WA, including to the firegrounds near Perth at the weekend.

-with AAP