Four Hector's dolphins have been killed by commercial fishers in the South Island this summer.
Fisheries New Zealand received two separate reports of the dolphins being caught during trawl fishing off the east coast of the South Island in December.
In one case one dolphin died, in another, three died.
The deaths were extremely disappointing, inshore fisheries manager Steve Halley said.
An upcoming review of the Hector's and Māui dolphin threat management plan could include more protection for the species, he said. Public consultation was planned for this year.
He said Fisheries New Zealand would be investigating the catches to work with the industry to avoid them happening again.
Forest and Bird's marine advocate Anton van Heldon said the catches showed more needed to be done to stop dolphins being caught in trawl nets.
He said the organisation was appalled and disappointed that the dolphins were caught, and that they were caught in December, but the organisation was only finding out in February.
He said it showed the mammals were vulnerable to being caught in trawl.
He said the number of dolphins caught could be higher, as the figures relied on fishers to self-report.
Hector's dolphins were listed as nationally endangered, with an estimated population of 15,000.