Key Points:

Police believe a man missing after a boating accident in the Hauraki Gulf last night is unlikely to be found alive.

Another man was found cold but uninjured on Browns Island by police aboard their launch Deodar III this morning.

Police believe a wave swamped their five-metre fibreglass boat, which sank about an hour before dark last night.

The alarm was raised about 8.30am today.

The missing man is believed to be in his 40s and was wearing a lifejacket.

Police and rescue personnel searched two gulf islands today and another team started looking in the channels late this afternoon.

But there was little hope of finding the missing man alive.

Sergeant Craig Kennedy of the Auckland police maritime unit said both men wore lifejackets, but were unable to contact anyone for help.

"They didn't have a VHF radio, but by the speed by all accounts of how things happened, they may not have been able to use that in time anyway," he told Radio New Zealand.

Police and coastguard boats and helicopters from the Police Air Support Unit and Westpac Rescue have been involved in the search.

Westpac Rescue Helicopter duty crewman Greg Brownson said today's conditions were perfect with clear blue skies and clean water.

"We could literally see for miles. We could see plastic bags, sea weed and birds but unfortunately no missing fisherman in his life jacket," Mr Brownson said.

He said rescuers were also looking for debris from the missing boat but found nothing.

Mr Brownson said he was not aware of the type of boat but it was unusual for a fibreglass vessel to sink quickly unless the hull was ripped out.