Niwa marine biologist Dennis Gordon said the creatures were known as `crimson jelly' and were native to New Zealand and southeastern Australia.
Around the size of the eraser end of a pencil, Dr Gordon said they were common in summer.
"It doesn't sting, it's completely harmless. It's quite a nice little thing to have in a home aquarium actually.
`"t's not a true jellyfish, although it's distantly related to that group.''
Meanwhile, another parent said their son developed a rash overnight after swimming at Orewa Beach on Sunday.
They described the rash as similar to a microscopic jellyfish sting.
The parent, who declined to be named, said they had since visited their local GP for help with treatment. Their daughter also suffered from the jellyfish.
An Auckland Council spokeswoman said they hadn't received any reports from residents who had suffered rashes after swimming at Auckland beaches.
Auckland Medical Officer of Health Dr Simon Baker also said the Auckland Regional Public Health Service had received no recent notifications of jellyfish stings.
However, the Regional Public Health Service said the majority of stings from jellyfish at Auckland's beaches during summer were due to microscopic jellyfish known as Hydromedusae.
The rash they caused was known as sea bathers eruption and tended to affect areas of skin covered by swimwear rather than exposed areas.
In recent years similar reports have surfaced at this time of year in other Hauraki Gulf beaches including Okahu Bay, St Heliers, Kohimarama, Murray's and Mairangi Bays.
The tiny organisms were brought close to the shoreline by warm weather and onshore winds and often became trapped underneath swim wear, or in peoples' hair.
The rash can last up to two weeks and can be treated with calamine lotion, antihistamines and mild steroid creams.
