An emergency has been declared in Queensland's southwest town of St George with major flooding expected.

The town about 500 kilometres west of Brisbane is bracing for flooding on Tuesday and Wednesday after weekend downpours.

The Balonne River may peak at 12.5 metres on Thursday, according to the Bureau of Meteorology.

Towns across the state's southwest and southern interior have experienced flooding this week. (9News)

This height is about 1.5m over Balonne Highway which runs through the town.

An emergency declaration was issued for St George by Queensland Police on Tuesday morning.

Police have asked residents to avoid using personal watercraft in the river system, saying it is likely to increase damage and property loss.

Homes in St George are at risk of flooding and sandbags are being provided, says Balonne Shire Mayor Richard Marsh.

"Residents in these areas are asked to warn neighbours, secure their belongings and relocate as soon as practical," Cr Marsh said.

"Ergon Energy have advised that power outages will occur in this area once the river reaches 12m."

The town of Dirranbandi - about 90km south of St George - is also preparing for flooding forecast for Friday.

It could take weeks for the water to recede from some areas. (9News)

The declaration has been issued for the Balonne River from Beardmore Dam to the Queensland-NSW border - about 140km.

Beardmore Dam, north of St George, which supplies water to the region has been spilling for almost a fortnight.

The dam was on the brink of being empty at the start of February and filled in less than two weeks, according to Sunwater.

The dam is now overflowing after being at just six per cent of capacity earlier this month.

"Sunwater expects that inflows will provide more than 12 months' urban water supply to St George," Sunwater said.

Towns across the state's southwest and southern interior have experienced flooding this week.

Charleville, Mitchell and Surat have all had flooding after the recent downpours, and its effects are expected to be felt for weeks, according to the bureau.

"It will take a number of weeks for the waters to recede there completely," forecaster Lauren Pattie said.

"There are a number of roads which have been cut in that part of the world, and it will be a little while before they are open again."