The sighting of a rare white humpback whale in Cook Strait has been called "once in a lifetime" by the leader of the whale spotting team.

The Department of Conservation whale survey yesterday sighted a white humpback whale just after 9am in Cook Strait.

Project leader Nadine Bott said it was first spotted by an ex-whaler from a lookout point at the east head of the Tory Channel entrance.

A team on a boat followed the pod including the white whale, but the glare of the morning sun on the water meant the team weren't aware of what they were following.

"They didn't think the whale was going to lift its tail for an identification so the boat pulled up parallel to it and they realised it was a white whale," Mrs Bott said.

Mrs Bott was back at the lookout and processed the pictures of the white whale then looked it up online.

"We realised it was adult size, and there's only four in the world.

"One's in Norway, and we knew it wasn't likely to be that one, and there's two calves, so we think it's Migaloo from the East Coast of Australia."

Mrs Bott said pictures of the dorsal fin of the white whale spotted in Cook Strait and Migaloo were similar.

"We're 99 per cent certain it's Migaloo."

Migaloo was once thought to be the only whale of its kind.

A biopsy sample was taken to from the white whale, which would confirm whether it was Migaloo, Mrs Bott said.

"It's a once in a lifetime thing, that's for sure."

The whale survey is into it's fourth week and concludes on Friday.

"It's been really successful. So far we've seen 122 humpbacks with four days to go. Our best year was 106 whales in 2012, so by all accounts we're breaking records," Mrs Bott said.

Humpback whales are on their annual migration from Antarctic waters to warmer waters in the Pacific for breeding.