Swim warning as jellyfish numbers explode along Auckland's beaches

Swimmers are being warned to watch out for jellyfish as numbers explode along Auckland's beaches.

The Safeswim site, run by Auckland Council and Auckland Regional Public Health Service, is urging the public to "exercise caution" over jellyfish blooms.

NIWA marine biologist Diana Macpherson says jellyfish blooms occur when water temperatures rise, which causes an increase in the amount of food available for jellyfish to eat.

"Jellyfish numbers increase as a result, then prevailing winds and currents can gather them up into dense groups and strand them on beaches," she says.

Safeswim says they can cause stings and a rash - also known as 'sea bather's eruption' - a painful experience in summer.

"There are at least three types of jellyfish in New Zealand known to cause stings, but there are many types of jellyfish around Auckland's shores which are harmless," the website states.

"Microscopic jellyfish can sting. As they are so small and almost transparent, they can get trapped unnoticed in swimwear, or hair, while swimming. As a swimmer gets out of the sea, water drains from the swimwear and traps the jellyfish between the fabric and the skin, causing the stinging cells to release their toxin.

"The rash becomes itchy and sore, and can vary from mild to severe, lasting up to a week. Children and people with allergies may get more severe reactions and can become unwell for several days.

"Bluebottle jellyfish sometimes occur around our coast. They produce a nasty sting and people should stay out of the water if these jellyfish are known to be present."

Macpherson's advice if you're stung 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.