Twenty pilot whales refloated this morning after a mass stranding near Nelson have beached themselves again and will be euthanised, the Department of Conservation says.

Volunteers have been warned to beware the hazards of decomposing whales on the beach, by the department.

About 100 whales were successfully refloated at Farewell Spit in Golden Bay on the high tide this morning however DOC Nelson ranger Kath Inwood said 20 of the mammals swam back to the same spot the initial pod of 416 landed at on Thursday night.

Inwood said the 20 whales were in bad condition and any attempts to refloat them were likely to be unsuccessful.

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"The decision has been made to euthanise those 20 to put them out of their suffering. It is sad but it gets to the point where you don't want to prolong the agony for them."

Beached whales often die from dehydration, collapsing under their own weight, or drowning when high tide covers the blowhole.

The other 80 refloated whales have joined a pod of 200 and the group have swum 7km north up the sandspit out to sea where they are being monitored.

"The tide is still going out so the guys are out in the boats and just making sure they stay in the water and don't strand as the tide goes out."

It would be about 5pm before the outcome of the situation would be known, she said.

A human chain in the sea to prevent the pod returning to shore was now out of the water and volunteers had been warned to beware the hazards of decomposing whales because a build-up of gases inside the carcasses can cause them to explode.

Volunteers frantically attempt to bring whales on their sides upright. Photo / Tim Cuff
Volunteers frantically attempt to bring whales on their sides upright. Photo / Tim Cuff

"The volunteers are moving off the beach because there's nothing more they can do for now."

DoC spokesman Herb Christophers said decomposing whales was "biology 101".

"Carcasses inflate when they start decomposing because gas can build up in the stomach. If you're nearby you'll get sprayed in whale guts to be honest. It's just one of those things."

Meanwhile, several Massey University pathologists had arrived on site to perform necropsies - the whale equivalent of an autopsy - on some of the 300 carcasses to try to determine the cause of the stranding, one of the biggest ever in the country.

On average there was a whale stranding once a year at Golden Bay.

It had been an emotional couple of days for the 500 DoC and Project Jonah volunteers, many of whom had named and sung to the whales they were caring for.

Christophers said many volunteers felt a connection with the whales they were caring for as the mammals whistled and clicked in communication.

"[The volunteers are] just ordinary folk. Just Mum and Dad and the kids. People get very involved with whale strandings. People have an affinity with whales," he said.

"Some people were naming the whales. Others are singing to the whales. The people have been sitting there comforting the whales."

The mass pilot whale stranding at Farewell Spit, Golden Bay. Photo / Tim Cuff
The mass pilot whale stranding at Farewell Spit, Golden Bay. Photo / Tim Cuff

Photographer and Project Jonah marine mammal medic Tim Cuff said when volunteers left the whales for the night on Friday many were upset.

"One German girl [Lea Stubbe] didn't really want to leave her whale. She was crying and had her hand on it. She was just kind of connecting with it."

Cuff said the dedication of volunteers, many of whom were young foreign tourists, was inspiring.

"It's always a pretty heart-wrenching scene. It's a pretty sad scene up on the beach where there's a long line of dead whales. The volunteers worked really hard."

Local iwi had performed a karakia over the dead whales and a decision over whether the bodies would be towed out to sea of left to decompose in the sand dunes was still to come.

The story so far

• Thursday night:

400-plus pilot whales beach themselves at Farewell Spit, at the northern end of Golden Bay near Nelson.

• Friday morning: The whales are discovered; 300 have died and 100 are cared for by 500 volunteers organised by Department of Conservation and Project Jonah.

• Friday: A small number of the whales are refloated while volunteers begin to become emotionally attached to the remaining surviving animals.

• Friday night: Whales are left alone on the beach and it's hoped a high tide will refloat some.

• Saturday morning: 300 volunteers turn out for a mass refloating of the whales on an 11.30am high tide. The refloat is successful.

• Saturday lunch time: A human chain of 100 volunteers stand in the water up to their necks to prevent the pod and another one with 200 whales from returning to shore.

Previous strandings in the area

2011:

Up to 80 whales beach at Farewell Spit.

Feb 2015: 200 whales stranded in the exact same location as today.

Feb 2017: 416 whales stranded at Farewell Spit in one of the country's worst mass strandings.