Freedive world champion William Trubridge has put pen to paper in a bid for more action to protect New Zealand's endangered dolphin species.

Trubridge has written an open letter to the fishing industry, pointing out New Zealand has faced criticism for the number of Hector's and Maui dolphin deaths in their fishing nets.

There are between 12,000 and 19,000 Hectors dolphins left, with one sub-species, the Maui's dolphin, down to just 63 adults.

Maui is the world's rarest and smallest marine dolphin, only found off the west coast of the North Island in relatively shallow waters.

The rest are the South Island Hector's dolphin.

Trubridge writes that he's concerned these dolphins remain without full protection, that bycatch continues at a rate greater than what's reported, and that fishing will eventually render the species extinct.

"Even if there was only one dolphin death every five years then it would still be an untenable pressure to the population, as demonstrated by peer-reviewed population studies, which show the population can support one human-induced mortality per 10 to 23 years," he writes.

Late last year, leading seafood companies Moana New Zealand and Sanford pledged to do to more to protect dolphin populations, announcing that both operations would end coastal set netting and end conventional trawling methods in the Maui dolphin habitat.