The Bureau of Meteorology has issued a Flood Watch for the Wilsons and Richmond rivers as another trough moves across Australia's mainland towards the east coast.

Jonathon Howe from The Bureau said Western Australia's Ex-Tropical Cyclone Dianne is the cause of the expected rainfall as a surface trough extends into western parts of Queensland, dragging a lot of wind and moisture into the system.

Showers and storms will push into northern NSW by this evening, and we will see heavy falls on Wednesday.

"It is moving quite quickly, though, and so by late Wednesday into Thursday morning, those showers and rain will gradually clear from northern NSW," Jonathon said.

(The weather map shows high rainfall for the Northern Rivers on Wednesday. Photo: Bureau of Meteorology)

In certain areas, rainfall could exceed 100mm; however, the trough's movement could be further south or north, making it difficult to forecast accurately. At this stage the forecast for Lismore is for 20-70mm of rain.

As the river catchments are already very wet from March's rainfall (553mm has fallen at Lismore airport), the risk of flooding is high depending on what areas receive the heaviest rainfall.

The Bureau says that minor flooding is possible for the Tweed, Brunswick, Wilsons, Richmond, Bellinger, Nambucca and Upper Macintyre Rivers.

Renewed minor flooding is also possible in catchments where flood warnings are already in effect, such as the Orara and Clarence Rivers and the upper reaches of the Gwydir River.

Catchments in the Flood Watch area are wet, and tides are forecast to be close to or slightly higher than the highest astronomical tide of the year along the coast of the Northern Rivers and Mid North Coast.

This weather system has the potential to cause minor flooding in the Flood Watch area from Wednesday. Flood Classes (minor, moderate, major) are only defined for catchments where the Bureau provides a flood warning service.

Catchments likely to be affected include:

• Tweed and Rous Rivers minor flooding

• Brunswick River and Marshalls Creek minor flooding

• Wilsons River minor flooding

• Richmond River minor flooding

• Clarence River minor flooding

• Orara River minor flooding

For the latest flood and weather warnings, see www.bom.gov.au/nsw/warnings/

For the latest rainfall and weather forecasts, see www.bom.gov.au/australia/meteye/

For the latest rainfall and river level information, see www.bom.gov.au/nsw/flood

You can click on Weather & Travel on the Lismore App or the website to easily follow the BOMs forecast.

Flood Safety Advice:

This Flood Watch means that people living or working along rivers and streams must monitor the latest weather forecasts and warnings and be ready to move to higher ground should flooding develop.

Flood Warnings will be issued if Minor Flood Level is expected to be exceeded at key sites along the main rivers for which the Bureau of Meteorology provides a flood warning service.

Severe Weather Warnings will be issued or updated if very heavy rain is forecast or observed.

For more information on the Flood Watch Service: http://www.bom.gov.au/water/floods/floodWarningServices.shtml

FloodSafe advice is available at www.ses.nsw.gov.au

For emergency assistance call the SES on telephone number 132 500

For life threatening emergencies, call 000 immediately