The parents who let four children aged 11 and under out in a boat by themselves are unlikely to be held to account.

John Llewellyn said he had not been contacted by authorities in relation to the incident in the Bay of Islands on Sunday, but in any case he didn't know who the family was.

The children's vessel, a 16ft rigid inflatable boat, broke down in the Te Puna Inlet after the fuel tank on the motor was filled with diesel instead of petrol.

Llewellyn was out fishing with a friend when he heard a Coastguard call about a boat in trouble. He found it about an hour later with the kids on board.

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"Those kids should not have been out in that boat in the first place, especially not knowing the difference between diesel and petrol, and definitely not at 11 and 8," Llewellyn said.

"They could've easily been looking at four corpses."

Northland harbourmaster Jim Lyle said had heard of the incident, but had been given little information about it. He confirmed a child of 11 was not legally allowed to operate a motorboat by themselves.

"Our bylaws are that if a boat does 10 knots, you have to have a person over 15 on board."

Lyle said there were children who knew more about boats than many adults, but there were reasons they should not be left alone in charge of a boat.

"It's not necessarily an age thing, but kids are more likely to make mistakes so that's why the bylaw's there."

The children had lifejackets in the boat - although they were not wearing them when they were rescued - but no VHF radio.

The maximum penalty for a person under 15 taking charge of a power-driven vessel without supervision, or allowing a person under 15 to do so, was a $200 fine.

"Is that what their lives are worth?" Llewellyn asked, when told of the penalty.

He saw the mother briefly when taking the kids back to her, but said she didn't seem to recognise the danger the children had been exposed to.

It was illegal to leave a child under 14 alone without making reasonable provision for their care and supervision.

Police also said they were not following up on the incident, saying it was the domain of the harbourmaster.