The author of New Zealand's longest running marine mammal survey says the creation of a sanctuary in Banks Peninsula has saved the Hector's Dolphins.
Liz Slooten has been cataloguing the dolphins there since 1984.
In the 80s the study found the population was nose-diving and there was only a seven percent chance of it recovering.
Slooten said the population is now stable with a 41 percent chance of recovering.
"That's due to the creation of the Banks Peninsula marine mammal sanctuary which was established in 1988 with a ban on kill nets out to four nautical miles off shore.
"But if we want the population to actually recover then we need to extend that connection."
She said new protection measures came into place in October - so they will be able to measure if they help.
Slooten who started the study during her PHD studies said it's unusual for a study to go for so long.
"We are now getting to the point where a dolphin which was born at the start of the study would have died by now and so we are starting to get one full life cycle for these dolphins where we are starting to see what age do they start breeding; how old do they get. All sorts of biological information that it's hard to get any other way," Professor Slooten said.