The trial for a charter boat company charged over the death of a South Auckland father has begun in the Auckland District Court.

Tevita Kava fell overboard during a 30th birthday party cruise on the Waitematā Harbour on June 3 2017.

Witnesses said he was standing at the back of the Red Boats charter vessel when a ramp gave way and he fell backwards into the water.

The body of the Mangere father-of-one - who could not swim - was found in water near Te Atatū nine days later.

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In June last year the charter boat company was charged under the Health and Safety at Work Act with failing to comply with duty that exposed Kava to risk of death or serious injury.

The company pleaded not guilty to the charge and a trial began shortly after 10am today before Judge Noel Sainsbury.

The trial is expected to run for four days.

Prosecution Sam Lowery opened the trial outlining Kava's death.

"Mr Kava was attending a friend's birthday party on the boat … the incident happened when Mr Kava entered the area at the rear of the ship called the duckboard," he said.

There was a BBQ on the duckboard and partygoers congregated there as one of them cooked, Lowery said.

That was "contrary to company policy".

When the BBQ was not in use the duckboard area was "off limits" and passengers were prevented from accessing it by a plank and steel chain.

When the BBQ was in use, access was restricted.

There was no signage in place stating passengers were not to go into the area.

Crew members had earlier moved passengers away from the area - where the "principal hazard" was a boarding ramp.

Lowery explained that the hinged ramp was fixed to the boat near the duckboard to assist passengers on and off the boat.

The ramp was in an upright position.

"The prosecution case is that Mr Kava leaned back on the boarding ramp and that it gave way. He fell backwards into the sea … tragically he could not swim and drowned," he said.

Lowery said it was up to Judge Sainsbury to determine what failings led to Kava's death.

Maritime NZ claimed the company failed to have adequate systems to ensure the ramp could not be inadvertently dislodged when it was not in use.

It also failed to prevent passengers from accessing the duckboard area.

Judge Sainsbury was shown photographs of the ramp and the mechanisms in place to secure it.

Lowery said the mechanisms - latches - were inadequate to prevent it from being dislodged.

"It was the ramp that ultimately gave way when Mr Kava leaned on it, causing him to fall backwards into the sea," Lowery told the court.

"When he did so passengers raised the alarm and the crew implemented man overboard procedures.

Lowery said there was no issue with those procedures.

However Kava disappeared under the water quite quickly and could not be found.

Seven witnesses will be called for the prosecution, including crew members from the boat and passengers.

One passenger will give evidence about seeing Kava, who was intoxicated, lean on the ramp and fall into the water.

Another was responsible for the BBQ and will also tell the court what he saw when Kava fell.

Lowery said three passengers ended up in the water after Kava fell in.

One of them fell through the boarding ramp.

All three made it back to the boat.

Defence lawyer Neil Beadle said company director Andrew Somers was supported in court by his family.

He wanted to acknowledge on behalf of his client how tragic Kava's death was and the impact it had on his family.

Beadle said the claim that the boarding ramp safety mechanism was inadequate was incorrect.

He indicated more would be made of that point later in the trial.

Yesterday, Kava's cousin Ngaire Speedy spoke to the Herald about the family's loss.

"We are slowly getting there," she said.

"With my mum - she was quite close to him, he was like another son to her - she still finds it hard.

"When she finds old photos of when we were kids she still gets upset."

Speedy said her family had unanswered questions about Kava's death and hoped they would be answered.

"I think that's what my mum is finding hard the most," she said.

"She knows he's gone, she just doesn't know why or how.

"Just why it took so long for the workers to react when the people that were on the boat said that he fell off."

Speedy's view was that if there had been a more rapid reaction to Kava's fall he would have been found sooner and the family would have been spared the ordeal of the wait.

"We don't want what happened to us to happen to someone else, to another family."