Freedive world champion William Trubridge is back on New Zealand shores as his partner prepares to give birth to their first child - very fittingly in a water birth.

Trubridge and his partner Sachiko Fukumoto - an actress and also an avid diver - flew in from Tokyo on Wednesday.

Fukumoto, who's originally from Okinawa, in Japan, said the pair have already found out the gender of the baby - she would be giving birth to a girl.

They'll base themselves at Trubridge's family home in Havelock North, Hawkes 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.

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Trubridge said they originally wanted Fukumoto to give birth in the sea, but were deterred by logistics - by Fukumoto's due date, New Zealand's waters would be too cold.

"Because it's in March, we'd have to be somewhere that it's warm," he told the Weekend Herald.

The water off the New Zealand coast will not be warm enough for a coastal birth.

The fallback was a birthing pool at the Trubridge home in Havelock North, where the 38-year-old's family is based.

Trubridge, who was born in the UK but moved to NZ when he was 18-months-old, holds 18 world records in various dive disciplines.

He set his latest record in 2016 when he plunged 122 metres into a blue hole in the Bahamas; beating his previous record.

He mainly competes in depth varieties of diving - which he also teaches through a freediving school, Vertical Blue, in the Bahamas.

It's been a busy few months for the pair, and Fukumoto was hoping the shift would provide a much needed break.

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"I've been travelling and working as though I'm not pregnant," she said.

"I was basically flying everywhere with this big belly. Just recently I told myself that I really need to stop and get ready for this baby."

Fukumoto has recently been in Japan for the filming of a French movie titled Women, directed by documentary maker Yan Arthus-Bertrand.

Trubridge had also been busy shooting various projects, teaching in Hawaii and as always, training.

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.

His parents had a house in Mahunga, on the Mahia Peninsula, 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.

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

For Trubridge, one of the best things about being back in New Zealand waters was the way the fisheries were managed.

"Most cases are pretty good - most fish stocks are doing okay in comparison to other places around the world.

The fishing trips would be short-lived - following their stint in New Zealand, Trubridge would get back into training for the AIDA World Championships in September.

"It's my career, and the way I support my family, so I need to maintain that," he said.

As for their baby girl, the pair said they're keen to get her used to the water in her formative weeks and months.

"It would be nice to put her in contact with the water and see how she responds - if she likes that sensation," Fukumoto said.

"If she has contact with the water from when she is a baby, there's a big chance she will like it," Fukumoto said.

New Zealand appealed as a spot to give birth, she said, because of the choice our health systems offers.

Fukumoto said it was "beautiful" to have the choice to take a natural route - and wanted women in Japan to know there was an option there for them, too.

"The main purpose is to send a message to Japan. I feel like we had no choice there around how we give birth."

The pair would also be shooting footage with a New Zealand film crew during their stay, for a video project revolving around the childbirth.