New Zealanders face an influx of jellyfish blooms this summer as warmer sea temperatures bring large population growth.
NIWA marine biologist Diana Macpherson said in a statement jellyfish blooms can invade beaches and sting swimmers, which is more common when sea temperatures rise.
"Forecasters have identified a marine heatwave in the waters to the north of the North Island and are keeping a close eye on sea surface temperatures around the country which have all been above average since October."
As a result, Macpherson said jellyfish numbers increase, with winds and currents stranding them on beaches in dense groups.
"They have no brain, no bones, and no heart but they reproduce, eat, and defend themselves or catch prey with astounding stinging cells called nematocysts, which work a bit like a harpoon that contains venom.
"In the event of being stung by a jellyfish is to flush the area with seawater to remove the stinging cells, carefully pluck off any tentacles that might be stuck on, then apply heat to relieve the pain and deactivate the venom," she said.
Auckland Museum marine curator Wilma Blom told Newshub in 2016 swimmers should avoid the water if there are jellyfish swarms.
"When they're out in the water and they still have those trailing tentacles, which can get quite long, metres possibly, and they carry all the stinging cells, and those are the bits you want to beware of basically."
Blooms can also impact industries such as salmon farms. NIWA says overfishing and pollution are worsening the problem.
For tips on jellyfish stings, visit the Ministry of Health's website.