A once-in-a-decade swell is set to hit the WA coast on Friday with waves between 40 and 50 foot expected in some areas.

The biggest waves are tipped for Margaret River, which should have big-wave surfers flocking to the region this weekend.

The swell beginning to build at Surfers Point, Margaret River. Credit:Amy McKie

Bureau of Meteorology duty forecaster Paul Vivers said the swell has been on the rise throughout the week and should peak on Friday around five to six metres.

"This will also see bigger sets of up to twice that size [12 metres] in certain places and means we could see some 40-50 foot waves," he told Fairfax Media on Thursday.