Up to 200mm of rain is forecast to fall in parts of Queensland in coming days, just weeks after powerful ex-Cyclone Debbie brought destruction to the state.

A low-pressure system moving east is expected to combine with a pressure trough tomorrow and inundate about 90 percent of the state.

"In parts of Queensland, it looks like there is potential for 200mm of rain over a couple of days," Weatherzone Meteorologist Brett Dutschke told 9news.com.au.

"That would be nearly four times the monthly average for May, with the wettest areas likely around central coast."

Heavy rainfall is expected in most of the country over the next three days. (9NEWS)

The Bureau of Meteorology has issued a flood watch in Queensland from Cairns to Gladstone, which includes Mackay, Townsville and Rockhampton.

"With any significant rain there is the possibility of riverine and flash flooding, however we are expecting rainfall totals and flooding to be generally lower than those following ex-Tropical Cyclone Debbie,” BoM Senior Meteorologist Scott Williams said

Rain and isolated storms are expected in southeast coastal districts including Brisbane on Friday, with decreasing rainfall in the 25 to 75mm range, and isolated showers and storms could continue into Saturday, Mr Williams added.

"We'll see a lot of thunderstorm activity develop through western Queensland and extend down into western New South Wales, “ he said.

The rain is expected to begin today on the tropical north coast and will spread to wider areas tomorrow and Friday.

In South Australia, today is expected to be the wettest day, although Mr Dutschke said the state will avoid any significant rainfall.

"It's going to bring rain to virtually all of South Australia, Tasmania, Victoria, NSW and Queensland," Weatherzone Meteorologist Brett Dutschke told 9news.com.au.

"The system is slowing moving east and the further east it goes effect the heavier the falls are going to be.

Victoria will cop the worst of the weather system on Friday, with heavy drenchings expected in the north of the state.

In NSW, the hardest hit areas will be in the west where up to 20mm of rain will fall between Friday and Saturday.

Mr Dutchske said there is a chance of some minor flooding in Northern Victoria and parts of NSW.

Tasmania could also have minor flooding in the north of the state on Saturday.

"Northern Tasmania on Saturday looks set to get 30 to 50 mm. There could be some falls there that get to 80 to 100, particularly about the higher ranges,” Mr Williams advised.