The sweltering hot weather has brought swarms of jellyfish and similar creatures into New Zealand waters.

Swimmers have reported dense swarms of creatures rubbing against their bodies as they move through the water.

Dennis Gordon, who is a marine authority at the National Institute of Water and Atmospheric Research, or NIWA, said many were not real jellyfish but a related species known as salps, which was harmless.

But others included the Lions Mane jellyfish, which was red, with poisonous tentacles several metres long.

Dr Gordon said the species was affected by the weather.

“It’s probably a consequence of the warmer temperatures, which speed up growth of plant plankton and that’s felt up the food chain to the things that prey on plant plankton – animal plankton, zooplankton – and then the things that prey on these zooplankton – including jellyfishes and fishes.”

Dr Gordon said numbers of jellyfish varied depending on tides and currents as well as temperatures.

Swimming today at #OrientalBay? Notice thousands of barrel-like transparent critters? These are #salps, herbivorous zooplankton that feed on phytoplankton (& don’t sting!) and are important for marine ecology and biogeochemistry @niwa_nz #UCMsalps pic.twitter.com/UHQmZN9fzJ

— Moira Decima (@moiradecima) January 28, 2018

Meanwhile, Wellington surfer Nick Butcher had an unpleasant encounter with such a fish during an ocean swim off Oriental Bay in Wellington Harbour.

“I was just swimming along, as you do, and then splat – face first into what we call ‘the mothership of jellyfish’. It kind of freaked me out and it stung me quite a bit.

“It was definitely a face plunge into it … and then the sting occurred and I was like,’oh great’,” he said.

Mr Butcher said the sting lasted for two hours but left no mark.