Rāhui placed on the 145 dead whales decomposing at Stewart Island's Mason Bay

A rāhui or restriction has been placed over the 145 pilot whales that died in Mason Bay on Stewart Island at the weekend.

Ngāi Tahu has been working with the Department of Conservation following the mass stranding and planned to follow various cultural protocols, including placing a rāhui or restriction over the whales.

Awarua Rūnanga Dean Whaanga said it was a sad event, especially given the large number.

"We are aware that all the affected whales are dead and we wish that appropriate respect is given to these taonga.

"The reason for the rāhui was to deter people from going near the whales as they decompose, primarily for their own safety."

A Department of Conservation spokesperson said they did not know why the whales stranded and it could be caused by a range of things, including sickness, navigational error and weather events.

"By the time DOC staff were able to get to the remote site about half of the whales had already died and some were part buried in the sand."

There were two pods stranded at the southern end of Mason Bay, approximately 2km apart.

A heartbreaking decision was made to euthanise the remaining whales due to their poor condition and hard to access location, DoC Rakiura operations manager Ren Leppens said.

"Sadly, the likelihood of being able to successfully refloat the remaining whales was extremely low. The remote location, lack of nearby personnel and the whales' deteriorating condition meant the most humane thing to do was to euthanise."

Niwa cetacean biologist Dr Krista Hupman said New Zealand had one of the highest number of strandings in the world, but also the highest rescue success rate.

It was unknown whether the large number of strandings was due to the high number of mammals in New Zealand or the environment, she said.

"You've basically got a piece of land which is basically sticking out and they think that maybe whales become trapped in there."

More strandings appeared to occur over summer and Farewell Spit seemed to be one of the country's most notorious region for strandings, which could be due to the topography of the region. Whangarei Harbour, Mahia Peninsula and the Chatham Islands were also prone to strandings.

Hupman said not a lot was known about why whales stranded, but they were usually coming onto the beach for a reason. Factors could include sickness, navigational error, geographical features, a rapidly falling tide, being chased by a predator, or extreme weather. More than one factor may contribute to a stranding.

"Whether they do become trapped or whether it's actually because one of the members of the pod is sick and then the other whales follow, we actually have no idea why."

Research is currently underway at Massey University looking at whether refloating pilot whales is the best thing for them or if pushing them back into the water was doing more harm.

In recent years, the biggest stranding of whales was at Farewell Spit in 2017 when 650 pilot whales were stranded.