A rare storm event has pummelled Western Australia's south, sending trampolines and sheds flying.

Perth and the South West are set to be pummelled by "tornado-strength" wind gusts and dangerous swells in "once in five-year" storm.

As the massive storm's set to bare down on parts of WA, the coastal town of Lancelin is tonight on high alert as huge swells threaten to breach its already eroded coastline.