Rare footage of a "white" southern right whale has been captured by Otago University researchers in the subantarctic Auckland Islands.
A white calf and its mother swimming side by side in crystal-blue water were among several whales filmed and photographed using drones over the course of a month.
The team, led by Professor Steve Dawson and Dr Will Rayment, studied the health of the species, which is "bouncing back" in numbers.
Rayment said the species was heavily impacted by whaling.
"They were almost wiped out from New Zealand altogether.
"A little remnant population hung on in the subantarctics and they've hung on since commercial whaling finished," he said.
Rayment said the high-quality aerial images captured mean they could track each of the whales through time.
"We can study lots of things about the population - like population size, survival rate, how frequently they come back and have their calves and this enables us to chart the recovery of that population."
He said they are also able to measure how "fat" they are.
"We can measure their body length and their body condition.
"We're looking to see how healthy they are, how healthy this recovering population is."
David Johnston, a Marine Science PHD candidate, said there are very few "white" southern right whales.
"It only occurs in about three and a half to 4 per cent of populations.
"It's due to the expression of a recessive gene and seems to only occur in males.
"They don't stay white throughout their lives. As they grow older, these individuals actually get darker."
Johnston said they never become completely black, which means they are to be easily identified as adults.
Southern right whales once had a widespread presence throughout much of the Southern Hemisphere. The New Zealand population was once at more than 30,000.
But over a relatively short space of time during the 19th century, commercial whaling led to a sharp decline in numbers, to fewer than 100 of the whales.
Since 1937, when commercial hunting for right whales was banned, the population has been bouncing back.
The most recent estimate is over 2000 southern right whales, increasing by up to 7 per cent year on year.