About 5 inches of rain fell in 12 hours, the Australian Broadcasting Corp. said. That was a record for Western Australia.
More rain was in the forecast.
"We're expecting that this level of rain that we're getting now could continue for up to 12 hours," Jason Whiteaker, a local official, said. "So we've got a long night in front of us, I think."
Collie, 132 miles south of Perth and 37 miles inland on the Collie River, is normally hot and dry during the Australian summer. Meteorologists described the rain as a 50-year storm, Perth Now said.
A storm Tuesday night caused flooding elsewhere in the state, including some areas in Perth.