Microscopic jellyfish is something that swimmers at popular Auckland beaches are being warned of.

Rashes that were likely to have been due to the stings of these fish have been reported by swimmers at Takapuna, Cheltenham, Narrow Neck and Read beaches, according to the Auckland Regional Public Health Service.

Itchy, red and a painful rash can be caused by the jellyfish, which are too small to be seen and can easily get caught inside togs.

Until 2008, it was thought that these rashes were being caused by sea lice and the health service says that not much is known about this fish.

Swimmers at other Hauraki Gulf beaches are also asked to practice concern.

According to Medical Officer of Health Dr Simon Baker, throughout the Hauraki Gulf amid warm summer weather, this problem is expected to spread.

He added, "The only sure-fire way of avoiding this rash is not to swim at affected beaches. We know this is the time of year that people want to be at the beach. We just want people to have the information they need to make choices for their own and their families' good health."

Dr Baker also added that the trick to avoid jellyfish is taking a shower post swimming, avoiding baggy clothes in water and removing togs.