A man suffered a head injury after a hammer incident on Kawau Island on Friday (file photo).

A man had to be air lifted to hospital after accidentally hitting himself in the head with a hammer.

The man, in his 20s, was thought to be briefly knocked out after the incident about 11.37am on Friday.

Auckland Rescue Helicopter Trust spokesman Lincoln Davies said the man was flown from Kawau Island in the Hauraki Gulf to Auckland City Hospital in a moderate condition.

He was believed to have a concussion.

READ MORE: Woman pinned under tractor on Motuihe Island

Davies said the man was not a Kawau Island local, but could not provide any further details.

JASON DORDAY/STUFF The man was air lifted from Kawau Island, about 40km from the CBD, to Auckland City Hospital.

The incident occurred on the busiest day on record for the helicopter trust.

Paramedics carried out 11 missions that day – more than on any other day since the trust was formed in 1970.

On the way to hospital with the Kawau Island patient, the helicopter was rerouted to Motuihe Island to treat a woman who had been pinned under a tractor.

Davies said a doctor and intensive care paramedic were dropped at the crash site before taking the Kawau Island patient to the mainland.

On the same day, crews transported a man who had been beaten in Whangamata, a baby boy suffering a medical complaint in Coromandel, and a man in his 40s who nearly drowned in Cooks Beach.

Helicopter call-outs usually spiked in summer, and the trust was highly likely to again round out the year with more than 1000 missions flown, Davies said.

"A lot of it is the fact that it's warmer weather, so a lot of people are out and about.

"We're really prepped for this being the busiest time of the year."