The Water Corporation is predicting the Wellington Dam may overflow by the weekend if heavy rain continues.
The dam is south-west Western Australia's largest water resource and is used for irrigation.
Heavy rains have increased the dam level by 40 per cent since July.
It is more than 96 per cent of capacity.
The region's largest drinking water supply dams, Harris and Stirling, are two-thirds full.
The corporation says eight of the 23 south-west dams are currently overflowing.