A teenager is recovering in hospital after suffering an anaphylactic reaction to what is thought to have been sea lice, after swimming.

The 17-year-old girl, of Auckland, was at Motuihe Island yesterday when she suffered a bad reaction after being in the water.

A St John spokeswoman said emergency services were called to the scene just after 10am.

The Auckland Westpac Rescue Helicopter landed on the island, in the Hauraki Gulf, at about 10.47am.

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A spokeswoman said: "It looks like she did have an anaphylactic reaction possibly to sea lice after swimming."

The teen, who was initially in a serious condition, was airlifted to Auckland City Hospital. On arrival, her condition had improved to moderate.

Motuihe is a recreation reserve controlled by the Department of Conservation. The island is popular for its campsites - also run by DoC - and short day trips.