Six whales have died on a remote beach in New Zealand’s South Island following a mass stranding and volunteers are racing against time to get 15 others that are still alive back to the sea.

Some 55 pilot whales washed up on Farewell Spit on Thursday. While most managed to make their way back out to sea, 15 have restranded and are now spread along about 1km (0.6mi) of the beach.

A video from Project Jonah, a non-profit working with marine mammals, showed volunteers pouring buckets of water on the whales to keep them cool.

“When the tide comes in, we’re going to have to move really quickly to bring these whales together, then move them out to deeper waters,” said Louisa Hawkes from Project Jonah.

Pilot whales are highly social animals and have a natural instinct to look out for one another.

Volunteers hope to bring the 15 stranded whales together in a “nice tight group” to help them refamiliarise with one another and swim out together, Hawkes said.

They will attempt to refloat the whales this afternoon, but time is tight. “We have to do all of that before the tide turns and drops again,” Hawkes said.

The group is calling for volunteers to help with the refloating.

New Zealand’s conservation department has deployed rangers, a boat and a drone to Farewell Spit to monitor any further strandings.

Mass strandings regularly occur at Farewell Spit, located on the northern-most tip of the South Island.

The conservation department describes it as a “naturally occurring ‘whale trap'” located along a migratory route for long-finned whales.

“Whales may be easily deceived and caught out by the gently sloping tidal flats and a rapidly falling tide,” it said in a statement on Thursday.

In February 2017, more than 400 long-finned pilot whales washed up there – the largest stranding in New Zealand for more than 100 years.