Thousands of kilometres of New Zealand coastline have been invaded by giant jellyfish, a phenomenon that has been linked to warmer sea temperatures.

In the last month mass jellyfish landings have been reported on beaches from Nelson in the south island to Whangarei in the top of the north island.

The Department of Conservation suspects more mass landings may have gone unreported or unsighted.

The jellyfish have been identified by marine biology expert Dr Lisa-ann Gershwin as Lion’s Mane – the world’s largest jellyfish – in Whangarei, and its close relative Desmonema in Nelson.

Gershwin, who is director of the Australian Marine Stinger Advisory Services, said the invasion suggested a significant bloom in the local jellyfish population, but the presence of them so early in the season was strange.

“I get excited any time there is jellyfish activity, and their welfare is not a concern because their population is not endangered,” said Gershwin, who has discovered 200 new species of jellyfish during the course of her career.

“New Zealand can probably expect to see more of these landings in the coming years, as jellyfish take advantage of prime breeding conditions in the oceans, which help them grow in super abundance.”

Gershwin said multiple factors could have contributed to a population explosion this season, but likely contenders included warming sea waters providing fertile breeding grounds, nutrient rich seas and a lack of natural predators for the juvenile jellyfish due to over-fishing.

Gershwin said any jellyfish landings in large numbers were always “interesting phenomena”, and gave scientists more opportunities to study the creatures up close.

Miles Lamare, a marine biologist at Otago University, said he suspected recent warm sea temperatures this year and the loss of natural predators from over-fishing were the most likely cause of New Zealand’s jellyfish invasion, although favourable winds and tides could also have contributed.

“This is likely something we will begin to see more frequently. Worldwide, jellyfish are also appearing in places they’ve never seen before, such as box jellyfish being found in Sydney harbour.”

Booming jellyfish populations have begun to be reported worldwide, including in Britain, the Mediterranean and Japan, with experts suggesting the increase in population numbers is strongly linked to human activities.

Andrew Baxter, the Department of Conservation’s marine technical advisor in Nelson, spotted hundreds of the Desmonema jellyfish lurking in the shallows of Tahunanui beach on an evening walk recently. He said the sighting was unusual, but the brown spotted jellyfish were a “stunning sight”.

Gershwin said New Zealand’s native jellyfish could give a painful sting, but were not life-threatening. She said the main problems they caused were disruptions to local fish farms.