The man, who wanted to only be known as Rod, noticed the rash yesterday morning in the areas where his children, Amelia, 7, and Thomas, 4, were covered by their togs and shirts.
They stayed in their swimming costumes for about half an hour after being in the water.
"It just suddenly reminded me of what happened last year," he said.
Rod has told Amelia and Thomas that it's "just a nasty rash" to avoid scaring them off going swimming at the beach.
"They're very itchy but they're being very tolerant about it."
Rod, of St Heliers, wanted the Auckland Regional Public Health Service to test beaches for the jellyfish - or hydromedusae - so parents had prior warning which popular swimming areas were affected.
But Medical Officer of Health Dr Simon Baker said the University of Auckland had been asked about the possibility and said there was no point.
It was just as effective to warn the public early each summer and give updates about affected beaches.
The rash tends to mainly affect areas covered by a bathing suit or clothes, because the transparent jellyfish get caught inside swimwear.
A tingling sensation is often noticed before the rash develops over several hours.
Children often suffer the worst reaction to the stings because of their soft skin, with hundreds to thousands of tiny red bumps forming in clusters.
Dr Baker said the health service and Auckland Council had received about 30 calls about the jellyfish.
It was likely that the problem extended all along the Hauraki Gulf.
Similar outbreaks have occurred the past two summers.
Dr Baker said the only guaranteed way to prevent being stung was to avoid bathing at affected beaches.
Advice to anyone who was stung was to remove swimwear as soon as possible and shower.
Towelling-down could cause the jellyfish to sting further because they released stinging cells when put under pressure.
Q&A: Microscopic stingers
What are they? Hydromedusae are microscopic and largely transparent jellyfish which can't be seen by the naked eye. Warm weather and onshore winds bring them close to the shoreline.
How does Sea Bathers Eruption occur? After the swimmer gets out of the water, the tiny organisms get trapped between swimwear and the skin, causing the stinging cells to release a toxin into the skin. A rash erupts where the swimwear clings to the skin, such as the buttocks or side of the torso. Children are most affected, likely because of their soft skin.
How can it be treated? Calamine lotion, antihistamines and mild steroids can be helpful. Typically, the rash lasts for a week or more. If the rash is severe or lasts more than a week, see a doctor for further treatment.
How can it be prevented? The only way to completely avoid being stung is by avoiding beaches where the jellyfish have been reported. During the summer, all Hauraki Gulf beaches can be affected. Other recommendations include not wearing baggy clothes in the water, smaller bathing costumes are better, and removing swimwear as soon as possible then showering. Thoroughly launder clothes worn during and after swimming.
