A 'ONCE in 50-years storm' gas dumped more than 100mm of rain on parts of Western Australia.

Harvey and the Mandurah-Rockingham area in WA's south were the hardest hit while thunderstorms today have left more than 2000 people without power.

The unseasonal weather across WA's south has given Mandurah its wettest December day on record, while heavy rainfall at Harvey - 109mm overnight - has been described as a "one-in-100-year event''.

The Bureau of Meteorology's senior forecaster Neil Bennett said more than 100mm of rainfall had fallen at Harvey, 130km south of Perth.

Bunbury, where residents are reporting it hasn't stopped raining since yesterday, has had 87mm and Collie had 110mm to 4.30pm yesterday.

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