Big sting: 15 people seek treatment after 'plague' of bluebottle jellyfish at Raglan beach

A bluebottle jellyfish washed up on a beach. Photo / File

Huge pods of bluebottle jellyfish have been wreaking havoc at a Raglan beach stinging dozens of unsuspecting oceangoers.

A Surf Life Saving Northern Region report said bluebottles had "plagued" Raglan's Ngarunui Beach on Auckland Anniversary Day, requiring minor first-aid assistance for 15 people.

Raglan Surf Life Saving Club patrol captain Isaac van der Vossen said there could have been at least 30 people stung through the day.

"We had guards swimming out the back saying there were huge pods of them, some with maybe a couple hundred jellyfish."

Most of the people seeking assistance were younger and had a bit a of a fright.

"It is quite painful, but it can be more the fright of it, while others won't even mention it, and will think it is all just part of it - mother nature."

Treatment involved removing the "stingers" and placing the affected area under hot water - as hot as the person could handle.

"That seems to take the pain away."

The stinging could last for a couple of hours, Van der Vossen said. If it persisted or got worse people should see a GP.

Van der Vossen said although they usually saw fluctuations in jellyfish numbers over summer it was normally at the end of summer and never in such large numbers.

"I have been here seven years and never seen that amount. Most summers we get them near the end of summer with the season change, and they typically hang around for a couple of weeks. It is all to do with the currents, the winds and what they are feeding on."

Niwa marine biologist Dr Dennis Gordon said bluebottle jellyfish were widespread around the world, but the science around why their numbers varied from place to place was limited.

"It is likely due to a combination of wind and current."

There were normally increases over summer as they reproduced shortly after the spring plankton blooms in September and October.

"There could also be a link to the warmer-than-average seas if that meant an increase in their food source," Gordon said.

They were also known to be more common on New Zealand's west coast, likely because of the prevailing westerly winds and currents, Gordon said.

However, it was unlikely to be linked to the swarm of bluebottle jellyfish that had stung thousands of swimmers in Queensland earlier this year, Gordon said.

Bluebottle tentacles could stretch up to 3m while in the water, and once beached their stinging cells could remain active for a couple of hours.