Mission Bay Beach in Auckland. Photo: RNZ / Jordan Dunn
Auckland Council is urging beach-goers to exercise caution after swimmers report itching, rashes, or red spots after entering the water.
The council's environmental health technical specialist Darryl Thompson said it was seabather's eruption - a rash caused by stings from the nematocysts - or stinging cells - of certain sea anemones and thimble jellyfishes.
However, National Institute of Water and Atmospheric Research (NIWA) emeritus researcher Dr Dennis Gordon told RNZ the rash could be caused by sea lice, a completely different type of organism, instead.
Gordon said seabather's eruption in Auckland was likely to caused by a siphonophorae.
"Divers call it the long, stringy, stingy thingy because it looks like long strings with little bubbles attached in the plankton.
"The little bubbles are little individuals because it is a colony, and they can break off in the seawater and get trapped between your bathers and your skin.
"The movement of your swimming togs when you're swimming causes these things to activate."
Auckland woman Tracy and her two daughters developed a rash after swimming at Takapuna Beach on Tuesday afternoon.
She told RNZ her daughter had said that she saw a jellyfish.
A rash on Tracy's daughter's torso after swimming at Takapuna Beach. Photo: Supplied
Hundreds of people replied to a Facebook post she made, having also developed a rash after swimming at various beaches along the east coast - including Martins Bay, Brick Bay Beach, Ōrewa Beach, Stanmore Bay, Long Bay Beach, Mairangi Bay Beach, Milford Beach, Castor Bay Beach, Cheltenham Beach, Narrow Neck Beach, Onetangi Beach, and Oneroa Beach.
As of Wednesday, Safeswim had only warned to exercise caution at Ōrewa Beach and Milford Beach.
However, further warnings had been issued for Takapuna Beach and Big Manly Beach on Thursday.
Another Auckland woman, Juliette, thought her daughter had chickenpox until she saw the post Tracy had made.
She told RNZ her daughter went swimming at Ōrewa Beach on Monday.
"I didn't notice anything unusual until Tuesday during bath time when she told me she had pimples.
"It was kind of on the areas where her swimsuit was. It was in patches, not really spread all over.
"At first, I thought it was the beginning of chickenpox, but she wasn't unwell, so I decided to wait and keep an eye on it. Then I found that post on Facebook, and it all made sense."
While Laura, who went swimming at Mairangi Bay Beach on Wednesday, said a pharmacist told her that it was sea lice.
"Our girl had red spots all over her torso, legs and arms. The pharmacist said that four people had been in this morning with the same."
Sea lice tend to bite uncovered parts of the body and leave itchy red dots on the skin, similar to a mosquito or flea bite. Sea bather's eruption, on the other hand, occurs under swimwear and can consist of hundreds to thousands of little red dots that may join together to form larger patches or appear as weals.
The Auckland Regional Public Health Service said children were the most affected by seabather's eruption.
The rash could vary from being mild to severe, and could last for a week or more.
It said calamine lotion, antihistamines and mild steroid creams could help. However, some children can become unwell with headaches, nausea and lethargy for several days, and would require treatment with steroid tablets or syrup instead.
