Hundreds of pilot whales have stranded on New Zealand’s remote Chatham Islands just days after a nearby beaching resulted in 250 mammals dying or being euthanised.

About 250 whales came ashore at Pitt Island/Rangiauria in the second stranding, taking the total number of whales stranded on the Chatham Islands to around 500, the general manager of Project Jonah, Daren Grover, said on Monday. The project runs a stranding hotline and mobilises marine rescues.

Because of the isolation of the Chatham Islands and an active shark population, it is considered too dangerous to actively refloat stranded whales. Unless they are naturally refloated by the tides, all of them will die.

“It’s highly likely that any whales that have survived the stranding will be euthanised,” Grover said.

“There simply aren’t the networks of people or the availability of people to be able to attend any refloat of any surviving whales out there. There’s a population of less than 40 people.

“It’s incredibly remote – one of the smallest self-contained populations on the planet. So the information of 250 whales stranding on their shores there, that’s a tragedy almost beyond imagination.”

The Chathams’ shark population makes attempting whale rescues highly dangerous.

“Great white sharks are known to frequent the coastal waters off the Chathams,” Grover said. “If we’ve got dead whales on the beach, or in the shallows, then they will attract sharks even closer to shore. So any human going out there attempting to be in among the whales and refloating, they are at a much higher elevated risk of being attacked.”

The reasons behind mass pilot whale strandings are not clear. The Department of Conservation describes the species as “prolific stranders”. The New Zealand strandings come less than a month after about 200 pilot whales died on the coast of Tasmania.

The Chatham Islands, 840km east of mainland New Zealand, are “a stranding hotspot, among the top three places for strandings in New Zealand”, according to the conservation department. The islands are the site of the biggest recorded stranding in New Zealand history – an estimated 1,000 pilot whales in 1918.

A department team was being mobilised in response to the latest stranding. They will probably be tasked with euthanising whales that remain stranded and in distress.

Over the weekend, conservation department workers had to euthanise more than 200 whales. A spokesperson told Stuff on Saturday that they “do not actively refloat whales on the Chatham Islands due to the risk of shark attack”.

The department said the surviving whales were “euthanised by the trained team to prevent further suffering”, with Māori and Moriori tribes [iwi and imi] present to offer support.