Novice rescued by boaties after mate left to go fishing 4km away.

An inexperienced diver was left alone at sea after his companion moved their boat more than 4km away to go fishing.

The diver would have had to battle a strong outbound tide to get back to shore and was fortunate two boaties chanced upon him floating in the Hauraki Gulf.

The "idiotic" pair have been blasted by the boaties and a diving expert, who said their actions were "crazy".

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The boaties were heading out to check their fishing net off Whangaparaoa Peninsula on Sunday afternoon when they spotted the diver.

"We set off from Gulf Harbour at about 2.30pm. We were about 2km off shore and we stopped to check our bearings on the GPS when I noticed a diver ahead of us," said one of the boaties, who did not want his name published.

The diver had no dive flag - a legal requirement in New Zealand waters. Aged about 19, the young man was struggling to tread water and was still holding his diving equipment when his rescuers got to him.

"I yelled out 'do you need help' and he put his hand up for the international sign for 'I'm in trouble'. We glided over, and I dragged him up on board.

"I asked him 'what are you doing out here' and he said 'my partner with my boat has moved off, gone'," the boatie said. "We thought he was joking. But he wasn't."

They offered to take the diver, who was still wearing his weight belt and holding on to other gear including a fishing spear, back to shore. However he said no, he wanted to find his boat.

They soon spotted it - 4km away with the diver's mate sitting on board fishing.

"We drove him over to his own boat and he started screaming at the other guy," the boatie said.

The boatie was furious at the other men and said if not for his boat passing by the diver would have died.

"There was nothing else out there. If we hadn't stopped ... he was breaking every single rule, every single one. There was nothing to tell boaties he was there and to slow down, we could have gone right over him. He had been out in the water a long time, it was the scariest thing, the most ridiculous thing I have ever seen."

Dive Centre Ltd managing director and experienced instructor Malcolm Kidd said the incident showed "sheer stupidity".

"They broke every fundamental rule of diving. It's crazy," he said. "There are so many things that could have gone wrong. Dive with a buddy - that's the golden rule. Both of these guys made major mistakes. They are just lucky that there was another boat around to pick the diver up."

Maritime New Zealand spokesman Steve Rendell said using a diving flag was a legal requirement for "obvious reasons".

Dive safety

• It is a legal requirement that a dive flag is displayed either by the diver or their companion onboard a boat. It must be able to be seen and identified from 200m away and must be clearly visible even when there is no wind. The flag means a diver is in the water, it informs other boat users to keep well clear and move at a slow speed. All other boat users must maintain at least 200m distance from the flag or keep their speed to less than 5 knots.
• Before you dive: get professional training.
• Have a medical assessment.
• Never dive alone.
• Maintain and service your equipment.
• Plan your dive and dive your plan.
• Carry a signalling device.