Two weather systems are colliding to bring thunderstorms, heavy rain and possible flash flooding across eastern Australia.

The troughs are passing over each other causing the wild weather in Queensland’s south and northern NSW, the Bureau of Meteorology says.

Rainfall totals up to 200 millimetres are forecast across both states in coming days.

Residents in an already sodden Queensland are being told to prepare for flash flooding.

Tweet from @BOM_Qld

The Darling Downs, Granite Belt and Maranoa in Queensland’s southwest are forecast to get thunderstorm activity on Thursday with widespread rainfall totals of 20-50 millimetres.

Isolated falls that increase the risk of flooding could be up to 100 millimetres in towns from Charleville to Goondiwindi and over to the Gold Coast.

There are flood watches for dozens of catchments across the state’s south-west, including the Paroo River, Wallam and Mungallala Creeks and Moonie River.

The bureau said floodwaters could rise on Thursday night.

For residents near the Moonie and Condamine rivers, it marks just a few months since river levels last rose, flooding homes in a January emergency.

The weather system will move south into northern NSW developing into a low pressure system, bringing widespread rainfalls of 30-50 millimetres and up to 100 millimetres in some areas.

The bureau’s Miriam Bradbury warned the system would bring severe storms with heavy rain, strong winds and high sea swells.

“As a trough deepens off the coast of east coast NSW it is likely to drag in further moisture and direct it across eastern and central NSW with the risk of heavy falls becoming more widespread an intense,” she said on Wednesday.

The New England and Northern Rivers regions are anticipating the first wave of wild weather before the system tracks further south to the Hunter, Sydney, Blue Mountains and Illawarra on Friday.

NSW State Emergency Services are preparing for the worst with residents urged to get ready for the storms.

“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,” assistant commissioner Sean Keans said.

A flood watch is in place for the NSW mid-north and south coasts, the Sydney region, and parts of the north-west.

Major flooding is possible on the Hawkesbury Nepean River from Friday.

-AAP