The risk status of the southern right whale or tohorā has been lowered as a new report shows the population around New Zealand is improving.

Photo: Steve Dawson / University of Otago

The latest Marine Mammal Threat Classification Report has revised the threat status of the tohorā from "threatened and nationally vulnerable" to "at risk and recovering" since the last report in 2013.

Department of Conservation marine mammal biologist Dr Dave Lundquist said in a statement that the data shows a continued recovery since industrial whaling ended with ongoing growth rates of 7 percent per year in the southern right whale population.

He said there had also been an improvement for sea lions or rāpoka, which had moved from being considered "nationally critical" to "nationally vulnerable". It reflected a stabilisation in the Auckland Islands' population since 2009 and increases in other breeding locations during this time, he said.

But Forest & Bird's marine advocate Katrina Goddard is was critical of the improved status for sea lions.

"There has been no recovery for sea lions. It's optimistic to say they're even stable," she said in a statement.

"The latest pup estimate was lower than last year. The threats haven't changed. Sea lions aren't improving - this is a species still in real danger. This is an illustration of how flawed the assessment criteria are."

The report shows the population of Hector's dolphins had also improved, although that was because estimates now suggested there were more of them in South Island waters than previously thought rather than because their population had increased.

"While the situation appeared to have improved in some locations for Hector's dolphins due to protection measures in place, the dolphins are still exposed to risk in much of their habitat outside those protected areas," Dr Lundquist said.

The Marine Mammal Threat Classification Report shows that 30 marine mammal species are listed as "data deficient".

"Marine mammals are still susceptible to impacts from human activities, so we need to be vigilant in reducing and mitigating these effects," Dr Lundquist said.

"We don't know much about many of the marine mammal species that live in New Zealand waters. We'd like to see more research in this area so we can better understand the threats facing these creatures and design work to mitigate them."