SYDNEY, April 12 (Reuters) - A tropical cyclone off Australia's west coast destroyed several homes and cut electricity to tens of thousands of people overnight before weakening in power on Monday morning (Apr 12).

Officials said about 70 per cent of the structures in the coastal town of Kalbarri, about 500km north of state capital Perth, had sustained damage when the category three storm made landfall late on Sunday.

Around 30 per cent of that damage was "significant", WA Emergency Services Commissioner Darren Klemm told the Australian Broadcasting Corporation.

"The situation in Western Australia remains very serious," Prime Minister Scott Morrison posted on his official Facebook page, adding the federal government's disaster response plan had been activated.

Tropical cyclone Seroja was downgraded after making landfall to a category two system and was forecast to continue to weaken throughout the day, although officials warned it would still bring potentially damaging wind gusts and heavy rain.

Photos on social media and local broadcasts showed downed powerlines, debris and houses stripped of roofs and walls. Western Australia state authorities opened three evacuation centres for displaced residents.

The region was on high alert for the storm, given that houses and other buildings were not constructed to withstand tropical cyclones, which usually do not push so far south.

"This is a rare weather event for people in southern and eastern parts of WA," the Bureau of Meteorology said.