The day after her ex-husband was killed on a dive trip, Nan Jiang called the boat's skipper to ask what happened - only to get through to a man who sounded relaxed about it all.

"Wasn't he on your boat?" she asked Zhenhua Yang, the man at the helm of his luxury yacht when its propellers struck his friend Wei Chen, killing him almost instantly.

Chen didn't know how to dive well and that's why he didn't come out alive, she recalled Yang saying.

"He has never contacted me, he never even sent a letter. He has never apologised," she told the court as Yang was sentenced at the Auckland District Court with a fine of $5850 for causing his friend's death.

His lawyer Paul Dacre KC also said he will pay emotional harm reparations of $150,000 to the victim's family members.

In court, Yang took the opportunity to finally apologise.

"I am extremely sorry to [Chen's] wife, his child, his partner, his parents, our friends," he whispered in Mandarin, wearing a face mask.

"Deeply sorry," his interpreter added in English.

On February 22, 2020, Yang took a group of his family and friends. including Chen, out for a scuba diving trip off Motutapu Island.

Yang's parents, fiancee and 7-year-old daughter were also on the boat.

They joined his friends Andrew Gan, a PADI dive master with a skipper ticket who has been driving boats for nearly 20 years, Gan's girlfriend who is a rescue diver, and Chen, who had just obtained his open water diving qualification at the time.

There was heavy rain and wind but the group went ahead with their dive plans.

Yang did not conduct a weather review before the trip, nor a safety briefing with the passengers.

He instead asked Gan, the dive master, to introduce his friends and show everyone where the life jackets and toilets were.

The sea was choppy and it was raining when the divers entered the water.

The divers split into pairs once underwater, with the two experienced divers pairing up, leaving Chen with another diver, Yung.

After about 20 minutes, Chen and Yung came up close to shore about 50 to 100 metres from the boat. Yang pulled the anchor up and drove towards them.

The two men were struggling to climb onto the boat using a nylon ladder with Yang's father and fiancee helping, instead of using a metal ladder built on to the rear platform of the boat.

Just then, sea conditions deteriorated, the wind and swell causing the boat to move towards the rocks, almost colliding several times.

Yang had moved to the helm of the vessel where he did not have a clear view of either diver.

He yelled at his fiancee to tell Chen and Yang to swim to either side of the boat but did not confirm if she had told them, and put the boat into reverse to move it away from the rocks.

At some stage while the boat was in reverse, its propeller struck Chen - but no one knew.

Yung had just managed to climb onboard when the boat crashed into the rocks. The group were looking for Chen in the water when the experienced divers Gan and Song resurfaced.

It was only then that they spotted Chen floating face down in the water.

An autopsy found he was killed by multiple blunt force chop wounds, the propellers cutting off his right arm and thighs, and cutting his flank and back.

Yang told police he thought the divers were already on either side of the boat and he had to reverse urgently because they were about to hit a rock.

He admitted he could not see what his fiancee and father were doing because his view was blocked by a sofa, and he did not wait to get confirmation that the divers were safely to the side before reversing.

Before the trip, Yang had completed a two-day course and maritime exam that focused on theoretical training in 2018, the year he bought the boat.

Chen was 39 and left behind his parents, a partner, and ex-wife Jiang with whom he has a 5-year-old son.

"Every day and every second of the day, my son must learn to grow up without a daddy," Jiang told the court.

- Qiuyi Tan, Open Justice