West Australian surfers from Margaret River to Gnaraloo are preparing for massive waves ahead of what is being billed "the swell of the decade".

Five- to six-metre surf buffeted by perfect offshore winds have unleashed on breaks across the coast this morning.

According to the Department of Transport buoy off Cape Naturaliste, the swell reached more than five metres with a very high 16-second swell period in the state's South West.

At Rottnest the buoy recorded swell of 3.5 metres.

Professional surfer and Yallingup local Taj Burrow said it was the biggest swell he had heard of.

"This swell's off the charts," Burrow told 720 ABC Perth.

"That storm out there was just so incredibly large and it's been tracking from a long distance, and it just looks like it gathered momentum. It's pretty exciting."

Waves breaking at the normally calm Gnarabup beach, near Prevelly, on Thursday night. ( Supplied )

The swell has been created by a strong low-pressure system 2,000 kilometres south-west of WA, which has been forming over the course of the week.

Bureau of Meteorology forecaster Paul Vivers said conditions would be exceptional.

"It's being called the swell of the decade ... I think that's a fair bet," Mr Vivers said.

"We've had some storm force winds out there ... so its generated what we call a very strong ramp for swell to occur.

"That system is moving into the bight at that latitude way south of the continent.

"But, as a consequence of the winds, we're going to get a long period swell coming up through the coast at a period of 18 seconds where normally the period of the swells is 12 to 13 seconds."

'It's about the biggest ride you can get': Taj Burrow

Swell period is a measure of the time between successive wave crests.

Taj Burrow, shown competing in the 2015 Margaret River Pro, says the swell will be "off the charts". ( Supplied: WSL/Kelly Cestari )

Burrow said it would not be a swell for tricks and high-performance moves.

"You're not looking for manoeuvres so much in swells like this. It's all about the biggest ride you can get and the biggest tube," he said.

The state's mid and north coasts were also expected to be battered by huge waves.

"Out at the Abrolhos Islands you'll be expecting swells of five metres on average, so you know there could be possible 30-foot waves out there occasionally as a result of this," Mr Vivers said.

"Probably up at Gnaraloo you'll see four or five metres.

"But there's going to be some really freak sets so the waves can be twice as big, so you might see some 30 or 40-foot waves."

He warned it would be not be a good day to be out fishing, especially rock fishing.

Geraldton surfer Tim Hornsby was expecting to see breaks working that normally did not have waves.

"Most spots it should only be for experienced surfers but also very fit surfers that sort of know what they're doing, otherwise I think you could get into a bit of trouble," Mr Hornsby said.