Swimmers are being told not be surprised if they find themselves swimming alongside a highly venomous sea snake related to the cobra.
The yellow-bellied sea snake - Pelamis platura - usually preys on fish, eating them head first after a fatal bite, and is one of the world's most poisonous snakes.
Bites to humans are uncommon, since its fangs are set back in its mouth, but its venom can cause paralysis or renal damage.
The snake has been sighted as far south as Cook Strait, but is most common in the northeast part of the North Island.
The National Institute of Water and Atmospheric Research, in a press release written to inform people about tropical animals which visit New Zealand shores, said swimmers shouldn't be surprised to see such animals swimming alongside them this summer.
Niwa principal scientist Malcolm Francis said the snake lived in the open sea and travelled with surface ocean currents.
"They live near the surface, hang around with logs and drifting seaweed, and feed on the fish that aggregate in those areas," he said.
Dr Francis, who was not available to answer questions on the subject, said all sea snakes were poisonous, but required a lot of provocation before they bit.
Sea snakes were news to senior lifeguard Luke Smith, who spent yesterday atop the tower at the Pauanui Surf Lifesaving Club on Coromandel Peninsula.
"We have not had any information at all, or any sightings; we have not even heard about it," Mr Smith said.
Swimmers had to look out for bluebottles, otherwise known as Portuguese man o' war, whose stings should be washed and treated with ice.
Last week a plane had spotted a shark in the vicinity, but there had been no sightings this week.
In a previous version of this story we described the press release as a warning to swimmers, but NIWA has issued no warnings.
- © Fairfax NZ News