A person was seriously injured after a boat submerged in Auckland.

A family heard screams of "help me, help me" after explosions on a boat in Auckland's West Harbour.

A man has been seriously injured. He was taken to Middlemore Hospital, a St John ambulance spokeswoman said.

Sharon Barnett was at her parents' home at Summerset Retirement Village in Hobsonville when they heard loud noises, which they first thought was something landing on the roof.

They then heard screaming coming from the harbour.

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"I heard the guy yelling 'help me help me'," said Barnett.

"We could hear people yelling back to him so knew he was going to be rescued."

Barnett said they drove to the scene on Ferry Parade on Herald Island and saw the man being taken away in an ambulance.

The boat has sunk.

Fire crews were called to the scene about 6.40pm after a fire was reported. When they arrived the boat was submerged.

The incident was understood to have happened on a private jetty, a Fire and Emergency spokesman said.

Greenhithe resident Tommy Cook said about 20 to 30 people - as well as four or five police cars, two ambulances, two fire trucks, a paramedic vehicle and a helicopter - were there when he arrived.

"It was a controlled scene. The police were interviewing people."

Cook said the jetty appeared to be in someone's back yard.

Several witnesses he spoke to told him the boat was badly damaged, describing it as "floating debris".

"Someone said [the explosion] sounded like a cannon going off and felt like a shock wave."

Eddie Grassic, who lives on Herald Island, said he heard "quite a big bang". A neighbour had told him a gas bottle was involved.