Nine hours, 22 kilometres and 930 dives later, Kiwi freediver William Trubridge has successfully swum the Cook Strait.

The freedive world champion left Wellington on Friday morning to swim to the South Island underwater using a special fin to raise awareness of critically endangered Hector's and Māui dolphins.

Speaking with Radio New Zealand, Trubridge said he was thankful to have completed it after battling challenging currents.

"I really didn't know what to expect because I've never even swum in that stretch of water before, let alone done anything like this, so I was going a bit blind," he said.

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"The whole time I was swimming, I was getting pushed different directions, north and south and then north again, but in the last stretch I was getting pushed sideways and almost missed the headland, and if I'd missed that one there was another current from the other side of the headland that could have pushed me straight out to sea, so I was lucky on so many levels."

Trubridge was hopeful his story would apply pressure on the government to revisit their fishing regulations and help better protect the endangered dolphins.

The 38-year-old and partner Sachiko Fukumoto recently returned to New Zealand shores as they prepare for the birth to their first child.

The couple 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."

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