Residents on Waiheke Island are being warned to take precautions when swimming at local beaches, following reports of people developing a spotty rash after swimming.

And it seems microscopic jellyfish - or Hydromedusae - are the problem, although many are attributing the rash to sea lice.

Dozens of locals have taken to social media in the last few weeks to warn their neighbours to stay out of the water when heading to the beach.

On the Waiheke Community Warning System Facebook page, members posted photos of bright red spots and rash-like symptoms on body parts.


Those who got the rash swam in beaches all around the island, including at Onetangi, Oneroa, Palm Beach, Little Oneroa, Shelly Beach (Putaki Bay) and Rocky Bay.

One woman said on Monday: "Swam at Onetangi after the beach races yesterday. Was heavily bitten by sea lice - as was the person I was swimming with. Just a warning. Now in pain and massively itchy ... ouch."

Another woman called on the council to put up signage around beaches so other families did not have to suffer what her family was now going through.

"I have a partner and child both covered head to toe in bites, with one of them starting to show an allergic reaction. [It would] be good to warn others how bad it is," she wrote.

On its website, the Auckland Regional Public Health Service said it regularly received reports of swimmers suffering from rashes often blamed on sea lice.

However, the authority said the majority of cases were caused by stings from jellyfish which led to a condition called Sea Bather's Eruption.

"Sea Bather's Eruption is an itchy rash which tends to affect the areas of the skin covered by swim wear, rather than exposed areas, after swimming in the sea," the ARPHS said.

"It is an allergic reaction to the toxin injected by the stinging cells of hydromedusae (the larval forms of jellyfish) which tends to occur after the person has left the water."

Sea lice bit people on uncovered parts of the body and the result looked more like sand-fly bites.

To prevent being stung while swimming, people were encouraged to wear speedos or bikini-style outfits.

Swimmers were also urged to remove swimwear and shower as soon as possible after leaving the water - and not to put the same swimwear on again until it had been thoroughly washed.

Auckland Regional Public Health Service said warm weather and winds brought jellyfish larvae into the Hauraki Gulf beach.

Medical Officer of Health Dr Simon Baker said sea lice were fish parasites and did not bite humans.

"It is almost certain that the rash under swimwear is caused by hundreds, even thousands of small jellyfish stings.

"When a bather drains their swimsuit, the jellyfish are trapped tightly against the skin, and their stinging cells release the toxin," he said.

Dr Baker said if a swimmer believed they had been stung, be sure to use the shower at the beach to wash off the jellyfish, rather than sit around in wet togs.

"A saltwater shower works best, but freshwater is almost as good. Then, change out of your togs as soon as possible."

He also suggested if stung, give togs a good wash at home as jellyfish can sting even when they are dead.

" is possible to get stung weeks after initial contact if your togs haven't been washed properly," he said.

Jellyfish stings could cause a mild itchy but painful rash that can last a week.

Children were also a common target for jellyfish, possibly due to their softer skin.

Calamine lotion, antihistamines and mild steroid creams such as 0.5% or 1% hydrocortisone may be helpful but if symptoms persist or worsen, people are urged to visit a doctor.

If visiting the beach, be prepared and take a change of clothes.

This may prevent stinging, or further discomfort and pain in the event of a sting, Dr Baker said.