Check the water before you enter is the message for Aucklanders, as stinging jellyfish wash ashore.

Auckland Council is urging the public to be cautious following reports of numerous lion's mane jellyfish, up to 8cm in diameter, being spotted on east coast beaches.

"The lion's mane jellyfish that have been washing up at different times and locations on east coast beaches, Waiheke and Great Barrier islands are not poisonous, however the public should avoid them when swimming due to their sting," said council regional environmental control manager Marcus Herrmann.

"In some cases they have been pushed into harbours and inlets in sizeable amounts - it's best to find an alternative swimming location on those days."

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Warm weather and onshore winds can bring jellyfish into beaches, which can appear at any Hauraki Gulf beach at any time.

However Herrmann said it is not clear exactly what is causing them to appear in such high numbers this year.

Concerns regarding jellyfish at beaches can be reported to the council on 09 301 0101.

Tips from the Council
• Keep an eye on dogs and young children to ensure their safety on beaches.
• If your dog begins showing signs of illness, take them to a vet immediately.

Medical advice
• Wash the affected area with fresh or saltwater.
• Remove any tentacles or stings attached to the skin - do not touch the tentacles or stings with your bare hands.
• Place the affected area in warm water.
• Do not apply vinegar, methylated spirits or alcohol, as these will make the sting more painful.
• If the reaction to the sting is severe or the symptoms worsen, antihistamines and steroid creams may be helpful. If the person has reduced consciousness or difficulty breathing call 111 and ask for an ambulance.