Bottlenose dolphin-loving boaties in the Bay of Islands are driving the friendly mammals away in an effort to get closer to them.
A Massey University report commissioned by the Department of Conservation (DOC) says the local bottlenose population has fallen from more than 250 to fewer than 100 in the past 15 years.
There's a risk the dolphins could be driven out of the area if numbers drop further.
DOC is now trying to work with boaties, iwi and the Bay of Islands community on ways to better manage boat interactions with the dolphins.
Although bottlenoses are not endangered around the world, the ones living around New Zealand are endangered because the three main populations are small and more vulnerable, DOC says.
Northern North Island director Sue Reed-Thomas says due to the high density of boats in the Bay of Islands, dolphins spend at least 86 percent of daylight hours around at least one boat.
"This is very disruptive for the dolphins. It means they spend far less time feeding, nursing their young and sleeping," she said.
The majority of interactions involve private boats and tourist operators, trying to observe the dolphins.
"It's very difficult to manage a group of wild animals swimming freely. The dolphins often swim towards boats themselves and you simply can't put a barrier around them or monitor every interaction they have," she said.
DOC has begun taking to local boat operators and iwi, and will be maintaining its moratorium on new commercial dolphin-watching operations in the region.