A teenager is in a comfortable condition in Whangārei Hospital after falling 7m down a rock face on an island where he was helping to clear weeds.

The Department of Conservation has confirmed the teen was part of a five-strong team working to clear islands in the Hen and Chickens chain, 22km east offshore from Bream Bay.

Rescuers say the teenager was probably saved from worse injuries because of his backpack.

Auckland Westpac Rescue Helicopter crew believe the backpack cushioned his fall but the teenager still suffered moderate injuries, including spinal, about 3pm on Tuesday.

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Because of the rocky terrain intensive care paramedic Rob Gemmell and Helicopter Emergency Medical Service doctor Heidi Baker had to be winched down to the patient.

The helicopter set down briefly on a shingle beach nearby but was forced to take off again as the tide came up.

The teenager was treated on the rocks, then winched up in a stretcher and flown to Whangārei Hospital.

DoC spokeswoman Abigail Monteith said the team had been on Whatupuke Island since Monday and the remaining members would continue to work until they were picked up today. They were not releasing any personal details about the injured teen.

Whatupuke Island, formerly known as Whakahau island, is composed of a large eastern section and a peninsula to the southwest. The coast of the peninsula forms one of the chain's main land features, a 300m wide bay called Starfish Bay.

The island covers 1sq km, and is steep, rising to 234m at its highest point.

Monteith said all islands in the Hen and Chickens group were designated reserves and a permit is required before landing.

People can be prosecuted under the Reserves Act for unauthorised acts on reserves with penalties of a prison term not exceeding two years or a fine not exceeding $100,000, or both.

Northland Emergency Services Trust general manager Vanessa Furze said the service had only one operational helicopter at the moment because the other two are undergoing maintenance.

Furze said the operational helicopter was not winch equipped, therefore Auckland Rescue Helicopter Trust was providing a backup service for the Northland region until the Northland-based winch helicopter was back in service.