Up to 145 pilot whales have died in a mass stranding on a remote part of a small New Zealand island, authorities said on Monday.

The stranding was discovered by a hiker late on Saturday on Stewart Island, 19 miles off the southern coast of the South Island.

Half of the whales were already dead and due to the condition of the remaining whales and the remote, difficult to access location, the decision was made to euthanise the remainder.

"Sadly, the likelihood of being able to successfully re-float the remaining whales was extremely low," said Ren Leppens, the Department of Conservation’s operations manager on Stewart Island.

"The remote location, lack of nearby personnel and the whales’ deteriorating condition meant the most humane thing to do was to euthanise.