A large lions mane jellyfish was found washed up on the beach at Devonport today.

Sue O'Callaghan estimated it was at least a metre across.

"We were just having breakfast on the wharf this morning and we came out and it was on the beach.

"We were rather surprised because of its size."

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Department of Conservation marine expert Clinton Duffy said lions mane jellyfish are common in the Hauraki Gulf over spring and summer.

"They reach a maximum reported size of 2.3m across the bell, so this is a very big one but not maximum size, although individuals this size are unusual.

"Large numbers of tiny juveniles can be seen during spring in the inner Waitemata Harbour but very few survive to this size.

The jelly fish have 70-150 thin, stinging tentacles which are difficult to see and may trail many metres behind the bell - up to 37m in the largest reported specimen, Duffy said.

"People should try to avoid them if possible. The severity of the sting depends on how much tentacle a person gets wrapped around them but there have been no reported fatalities, and the symptoms are described as temporary pain and localised redness.

"Fragments of tentacles can continue stinging after they have been broken off by the surf."

The main treatment if stung was to remove the tentacles. "Pull the tentacles off. Don't rub, it can trigger more stinging."