Days of heavy rain have polluted dozens of beaches around Melbourne's Port Phillip Bay, while rainfall totals above 200mm are forecast for eastern NSW and southeast Queensland from Thursday to Saturday.

The Darling Downs, Granite Belt and Maranoa in Queensland's southwest are set for showers and possible storms on Wednesday as the weather event intensifies on Thursday bringing widespread, heavy rainfall.

"Widespread shower activity could result in falls between 20mm to 50mm if we do see thunderstorm activity," the Bureau of Meteorology's Daniel Hayes told AAP on Wednesday.

Those rainfall totals could exceed 100mm in towns like Charleville, Roma, Dalby, Warwick, Toowoomba and Goondiwindi extending over to the Gold Coast on Thursday.

Brisbane will not escape the wet weather with potential falls up to 50mm on Thursday.

This could lead to flash flooding for the already saturated areas after months of soaking weather, Mr Hayes warned.

Premier Steven Miles said the state is only just recovering from the most recent floods so the next onslaught of rain heightens the possibility of more river rises, urging residents to be cautious and watch for warnings.

"I assure Queenslanders and the Queensland community that we're ready for whatever nature throws at us," he said on Wednesday.

The weather system will then track further south to the NSW coast as two troughs meet on Thursday.

But the wet weather will not stop there for Queensland, as a trough over the Coral Sea moves towards the northeast of the state on Friday bringing rainfall totals between 30mm and 40mm to the Mackay region over the weekend.

As for the rain band pushing south, the NSW State Emergency Service anticipates a flurry of requests for help as the trough soaks the state.

Communities in the New England and Northern Rivers regions, and along the northern half of the mid-north coast, should expect widespread showers of 30mm to 50mm and up to 100mm in some areas.

The bureau is forecasting the trough will deepen on Friday and potentially develop into a low-end east-coast low, impacting the NSW mid-north coast before shifting to the Hunter, Sydney, Blue Mountains and Illawarra by the evening.

"That may bring damaging winds, heavy rainfall and dangerous surf to much of the coastal area," bureau meteorologist Sarah Scully said.

The weather system is expected to move south on Saturday to the Illawarra and NSW south coast, bringing heavy rainfall, strong to gale-force winds and hazardous surf.

NSW SES assistant commissioner Sean Keans urged east coast, Blue Mountains and metropolitan Sydney communities to prepare for poor weather by cleaning gutters, trimming trees and branches and securing loose items around homes.

"We still have a few days until the weather may deteriorate, so it's a good time to make sure you take steps to prepare your homes," he said.

Emergency assets and crews have been pre-positioned ahead of the forecast wild weather, Mr Keans said.

"Flood and storms teams are on standby to respond should they be required, but we're pleading with the community to be prepared, stay informed and not drive through floodwaters."

The warning comes after Melbourne was hit by the equivalent of one month's rain on Monday, while some parts of Victoria recorded rainfall not seen for more than 150 years.

Australian Associated Press