Kiwi freedive world champion William Trubridge is attempting to swim the Cook Strait underwater to raise awareness for the endangered Hector's and Maui dolphins.

Trubridge, who returned to New Zealand shores earlier this month as his partner prepares to give birth to their first child, started the 22km swim today off the coast of south Wellington.

Coming up for air every 25 meters to take a 10-second breath, the swim will likely take Trubridge 10 hours to complete.

His father, David Trubridge, told Stuff they had done a successful trial run earlier and that this was just one of many projects Trubridge had worked on to draw attention to the Hector's Dolphin.

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"It's been caught in fishing nets, its range has been hit by commercial fishing. There's only about 30 of them left," said David.

"Our country need our dolphin, there's nothing else like it in the world, and that's why he's trying to draw attention to these dolphins and this event he's doing today."

Trubridge and partner Sachiko Fukumoto - an actress and also an avid diver โ€“ will be based at his family home in Havelock North, Hawke's Bay, for the next three months - spanning the last month of Fukumoto's pregnancy and the first couple of months after she's given birth.

Trubridge, who set a new freediving world record of 102m in 2016, said they originally wanted Fukumoto to give birth in the sea but were deterred by logistics.

While New Zealand didn't have ideal conditions to continue his training in the ocean, Trubridge was looking forward to a different kind of diving during his stay, equipped with a boat for spearfishing excursions.

"When I go out there, it tends to be full of fish that aren't accustomed to seeing people," he said.

"It's a great spot to go out and get Kingfish and other seafood to put on the table."