Residents in Northam, about 100km east of Perth, should be ready to evacuate due to rapid and powerful flooding in the Avon River, authorities say.

Residents in Northam, east of Perth, should be ready to evacuate as authorities warn fast flowing floodwaters will rise quickly.

The Department of Fire and Emergency Services told residents at 4.30pm on Friday to watch water levels, have a plan, and to create sandbags by filling pillow cases with sand and place them in toilets and over drains to prevent sewerage backflow.

Minor flooding is also predicted for the Perth metropolitan area, due to rising waters in the Murray and Swan River catchments as unusually heavy summer rain hits WA's south.