Up to 145 pilot whales died in a mass stranding on a remote New Zealand island at the weekend, authorities said on Monday.

The Department of Conservation (DOC) said two pods of pilot whales were stranded on a beach on Stewart Island, 30 km (20 miles) south of New Zealand's South Island.

Half of the whales were already dead when they were found by DOC officers, who were notified on Saturday night by a hiker camping in the area.

A decision was made to put the rest of the whales down due to their poor condition and the remote location, DOC Rakiura Operations Manager Ren Leppens said.

"Sadly, the likelihood of being able to successfully refloat the remaining whales was extremely low," said Leppens, who described it as the most humane thing to do.

"However, it's always a heart-breaking decision to make," he said.

New Zealand has one of the world's highest rates of whale strandings, although the precise cause for it is not known.

The DOC said it responds to an average of 85 incidents a year, mostly for single animals.

Many factors could contribute to such whale and dolphin strandings, including sickness, navigational error, geographical features, a rapidly falling tide, being chased by a predator, or extreme weather, the DOC said.

Ten pygmy killer whales were also stranded on Sunday at 90 Mile Beach on the western coast of the North Island. Two have since died and attempts were being made to refloat the rest.

The DOC said the two events were unlikely to be related.

About 300 whales died on a beach at the northwest tip of South Island last year in one of New Zealand's largest recorded mass whale strandings.