Multiple mass deaths of New Zealand fur seals on the Canterbury coast have been reported to the Department of Conservation this year.

Andy Thompson, the operations manager for DoC's Mahaanui/Sockburn office in Christchurch, said today's find of about 25 dead seals at Te Oka Bay repeated a pattern of the last four months in the vicinity.

The difference, however, was that the two earlier finds along the southern bays of Banks Peninsula, each of about a dozen dead seals, were of younger animals.

The dead seals were found at Te Oka Bay on the isolated southern side of Canterbury's Banks Peninsula. Map / topomap.co.nz
The dead seals were found at Te Oka Bay on the isolated southern side of Canterbury's Banks Peninsula. Map / topomap.co.nz

DoC staff would retrieve the dead seals from Te Oka Bay tomorrow, following the grisly find by members of the public.

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Several would be sent to Massey University in Palmerston North for necropsies (animal autopsies) in the hope of discovering why they died.

"When we've responded to the previous incidents we haven't found any reason to be suspicious of them. That said, we will go out there with an open mind tomorrow."

It hadn't been possible to analyse the previous deaths thoroughly because the animals were badly decomposed by the time their deaths were reported to DoC.

Asked what the possible causes of death in the three events might include, he said, "It could be anything from something deliberate, to viruses, to heavy seas. One of our marine specialists talked about an algal bloom even."

At the last count, in around 2014, it was estimated that around 12,000 seals were living around Banks Peninsula. The population is growing.

Today's find was made by a beach-walking couple at Te Oka Bay, which is 37km southeast of Christchurch.

Kerry Munroe told Stuff he came across around 25 "mums with pups" when walking with his wife.

"It's not what you're expecting to see, we just wanted to go out for a quiet, Saturday afternoon walk," he said.

"We're curious about Te Oka Bay and it was very pleasant until we came across that."

According to Munroe, no one else was at the beach. No other fur seals were around at the time of their discovery.

There were no gunshot wounds apparent on the bodies of the dead seals.

The witness had reported the find to the Ministry of Primary Industries, which said a representative would be sent to the beach.

Marine mammal expert Ingrid Visser said it was not natural to see such a large group of seals die together.

It was "highly likely" that foul play was involved, she said.

"It's not common to see a large number of seals to die clustered together like this.

"But of course experts would have to rule out natural causes," Visser told Stuff.

New Zealand fur seals are protected but not endangered.

All seals are protected under the Wildlife Act and Marine Mammals Act; breaches can incur up to two years' imprisonment and/or fines of up to $250,000.