An endangered tropical sea turtle was treated at Wellington Zoo's The Nest clinic after washing up injured and hypothermic on Wellington's South Coast on Saturday.

An endangered tropical sea turtle has only a 50 per cent chance of survival after washing up in the frigid winter waters of Wellington's Lyall Bay.

The 25-kilogram Olive Ridley turtle was suffering from hypothermia, dehydration and had a badly cracked shell when found on the beach on Friday night. Wellington Zoo's The Nest clinic was nursing the turtle back to health, but its prognosis was still "guarded," vet Lisa Argilla said.

"He still has a probability that he might not make it."

The only way staff could tell the turtle was alive yesterday was by touching its eyes, which moved in response. On Sunday it was still exhausted but lifted its head occasionally, reacting most when stuck with needles in its neck and flipper.

"He was protesting a little bit when I was taking blood," Argilla said.

"I used the smallest needle I could but everyone is a bit scared of needles."

Argilla and her staff kept the turtle warm and wet in a paddling pool overnight, fearing it would develop pneumonia and damage to its organs. On Saturday its temperature was too low to register, but it had reached a healthy 27 degrees by Sunday.

X-rays showed the turtle had reasonably healthy lungs, but its shell had a bad crack and the exposed bone was at risk of infection. It was given antibiotics and given an electrolyte drip in The Nest.

"He's been having an okay time but he's still very, very unwell."

Olive Ridley turtles were usually found in tropical or subtropical waters and this one might have been caught in a strong current from the Carribean, Argilla said.

"If you're from the Caribbean, why would you come to this could southerly place?"

The turtle was 10 to 15 kilograms underweight for an adult Olive Ridley so was likely to be a young specimen, although it was probably underweight too, Argilla said.

It was too sick to eat as its guts were in a frozen state, but staff would begin feeding it when it did a poo, she said.

The zoo had never treated an Olive Ridley turtle before, and only a few have ever been reported as washed up in New Zealand. Argilla used Google and an old vet manual to devise a treatment plan, she said.

"I didn't ever think I'd need the sea turtle husbandry manual, but I'm glad I had one."

Wellington Zoo was in discussion with Kelly Tarlton aquarium in Auckland to care for the turtle when it recovered enough to travel.

Breaker Bay woman Bev Hong and her son Isaac Leniston-Hong, 12, saw the turtle at Lyall Bay beach on Friday night. It was lethargic and some spectators thought it was dead until it moved its head, Hong said.

"It was really cold. I was thinking 'you're in the wrong place, buddy'. It was an amazing, gorgeous creature."