A team of scientists have captured what could be world-first footage of a pygmy blue whale feeding her calf.
The project's leader, Oregon State University marine mammal expert Leigh Torres, said the team was using a drone to measure the size of the whales when they became aware they were witnessing nursing behaviour.
Instead of swimming next to its mother, the calf was repeatedly going under her and staying there for sometime, Dr Torres said.
"The calf would sort of alternate going underneath the mum for a little while and then come back up to the surface to breathe and then go back under mum.
"We documented that for about 10 minutes so it was pretty remarkable."
Dr Torres believed the encounter, in the waters between Cape Farewell and Taranaki last month, was the first time the nursing behaviour had been documented on video.
"I think it is pretty rare but to be honest we had no idea we were watching it from the vessel, so we may have seen it in the field before but just not known.
"I think having this perspective, a bird's eye view - an aerial view - really really helped.
"It's certainly the first we know of documenting this sort of behaviour and I think it is really unusual."
On the same day the researchers filmed the encounter with the whale feeding her calf they also saw four other whales with calves.
"We are not entirely sure what it all means except we can say that mothers are reproducing and having calves which is a good sign."
In another rare encounter earlier in February the researchers spotted two male whales racing one another at speeds of up to 18 knots.
The behaviour was thought to be a form of male competition done in order to attract females, Dr Torres said.
The scientists have spent six weeks aboard the NIWA research vessel Ikatere this summer studying the whales' New Zealand foraging grounds between Farewell Spit and Cape Egmont.
Dr Torres said they had encountered about 30 blue pygmy whales, which can be up 20 metres long and weigh up to 100 tonnes, in that time.
The three-year study is a collaboration between the Department of Conservation and the Oregon State University.
There were just four confirmed blue whale foraging grounds in the Southern Hemisphere, outside of Antarctic waters, and it had been thought that blue whales were only passing through New Zealand waters while migrating.
"We want to know when and where the blue whales occur in the South Taranaki Bight, as well as how many blue whales use this area as a foraging ground," Dr Torres said.