Green turtles Oscar and Kiwa released at Poor Knights Islands

Harry Josephson-Rutter about to release Oscar at the Poor Knights Islands after 18 months rehabilitation. Photo / Kelly Tarlton's

It was a bittersweet moment as Harry Josephson-Rutter watched green turtles Oscar and Kiwa swim free at the Poor Knights Islands.

The "teenagers in turtle years" were released at Maroro Bay, a popular dive site known as The Gardens, on Wednesday after 18 months rehabilitation at Kelly Tarlton's in Auckland.

Oscar, known for his happy nature and huge appetite, was found on Ninety Mile Beach in July 2016. He was cold, underweight and suffering from infected fractures on his flippers.

Kiwa was suffering from dehydration and had a severe eye infection when also found on Ninety Mile Beach.

The man charged with nursing them back to health was Mr Josephson-Rutter who also released them back into the ocean.

"It was a bittersweet moment to see them go. We've seen them in their worst condition and then up to a point where they are healthy. After 18 months you build a bit of a bond with them but now they are in the ocean where they need to be.''

The turtles between them had gained 15kg since arriving at Sea Life, Kelly Tarlton's Turtle Rehabilitation centre.

When they arrived turtles were often too weak to feed themselves and needed to be tube fed. Once they gained some strength they were offered a variety of foods including fish, mussels and squid.

They spent little time milling around the boat on their release on Wednesday and quickly swam off to make the most of their new surrounds.

Oscar and Kiwa were fitted with a tag on their flipper so if they turn up anywhere else in the world they will be tracked back to Auckland. Some turtles are released with satellite monitoring.

Mr Josephson-Rutter said green turtles could live up to 100 years. Oscar and Kiwa were too young to determine what sex they were.

"Hopefully they have a long life and help grow their own population."

He is preparing three other turtles for release including a hawkesbill and a loggerhead, both found in Northland last year.

The hawkesbill turtle had eaten plastic but was fortunate enough to have had it pass through its system.

Plastic and other rubbish is the biggest danger to turtles and the common factor in those needing treatment at Kelly Tarlton's as most were sick from "gut compaction" caused by eating plastic.

Clarke Gayford, the partner of Prime Minister Jacinda Ardern and host of the Fish of the Day television series, helped to release the turtles and tweeted about the experience afterwards: "Perfect day indeed to release these rehabed turtles."