An Auckland yachtie was found "flopping around like a half-dead fish" in the Waitemata Harbour after a huge explosion blew his boat into pieces.

"I thought we were going to find a 100 pieces of him like the boat," said Herald Island resident Michael Reeves after rescuers and residents rushed into the water on Saturday evening.

The explosion - believed to have been caused by an LPG leak - ripped Mike Lawler's boat to pieces and sounded like a "sonic boom", said another resident, Mark Kayes.

Kayes called police immediately and said the explosion was so powerful, it blew out one of his windows.

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"'Who do you want?'" the voice at the end of the 111 call asked Kayes. "I said 'everybody'."

Reeves had also been put on high alert by the colossal noise.

"The explosion was huge. It felt like something had hit my house. I felt it go right through me. It was intense."

Reeves and his father rushed down to a jetty to find a nightmarish scene unfolding about 6.30pm.

The reconditioned boat that Lawler had called home for the past year had been destroyed.

A photo of the boat taken hours before the explosion. Photo / Elizabeth Kayes
A photo of the boat taken hours before the explosion. Photo / Elizabeth Kayes

There was nearly "nothing left" of the 11m boat, just pieces of debris being pushed around by the current as the wreckage sank away from sight.

"I thought we were going to find a 100 pieces of him like the boat," Reeves said.

Reeves said instinct and adrenaline took over as he jumped into the water to begin a frantic search for Lawler.

The massive boom had roused other Ferry Parade residents who called out to each other from private jetties offering help.

In the water Reeves and his father searched among the debris.

At first they could not hear anything, then he heard the moaning.

"He was shell-shocked. He was bobbing up and down as I got to him," Reeves said.

He was "flopping around like a half dead fish" and had suffered burns on his hands and face.

"But by the time we got him on the boat he was actually having a laugh.

"Everybody was saying he should buy a Lotto ticket."

It was incredible he was still alive, Reeves said.

"He shouldn't be, but he is."

A Givealittle page has since been set up for Lawler by Samantha Fels, a family friend.

On the page she said the explosion was caused by an LPG leak. She said Lawler's boat was not insured and he was looking at about $10,000 to remove the wreck from the seabed.

"On top of a long road to recovery and finding somewhere to live, he doesn't need the stress of trying to pay for the bills headed his way," she said.

Tracey Sharman was among the residents who raced down to a nearby jetty.

"It shook the whole island, let alone the boats in the water," Sharman said.

Sharman's son was among the boaties who had gone to help, and sitting on a dinghy near the wreckage she said he could see a gas cylinder float across the water.

Lawler appeared "extremely dazed" but was talking and could hear those around him, she said.

It was scary but people had reacted appropriately, she said.

Sharman's neighbour Trish Purchase had been living in the area for the past 25 years and said it was a given that boaties helped each other if there was any trouble.

Auckland Rescue Helicopter intensive care paramedic Russell Clark told the Herald yesterday his crew landed at Whenuapai Airbase shortly before 6.45pm.

Clark said Lawler was conscious when he reached them but suffered "serious burns".

The water had helped cool Lawler's burns, he said.

Clark had told him he was lucky to be alive.

"With that sort of blast, any sort of explosion, just the sheer forces involved can give you severe traumatic injuries," he said.

"He got off pretty lightly."