Partygoers on a cruise when Tevita Kava fell overboard have spoken of the frantic search for the father-of-one and what happened in the moments before he plunged to his death.

Kava fell from The Red Boats' charter vessel Reo Moana during a 30th birthday party cruise on the Waitematā Harbour on June 3 2017.

Witnesses said the Māngere man was standing at the back of the boat when a boarding ramp he was leaning on gave way and he fell backwards into the water.

Kava's body was found in water near Te Atatū nine days later.

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In June last year Maritime New Zealand charged the charter company 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 is defending the charge and is on trial at the Auckland District Court before Judge Noel Sainsbury.

Prosecutor Sam Lowery said Kava effectively died as a result of "inadequate" safety measures.

However, the company deny that is the case.

Yesterday the court heard from the boat's skipper Sam Somers, the son of company director Andrew Somers.

He was adamant that the ramp Kava leaned against had been properly secured.

He said Kava was in an area of the boat - known as the duckboard where passengers enter and exit and housed a boarding ramp - where he was not authorised to be.

Two other crew members gave similar evidence.

This morning Judge Sainsbury heard from two partygoers about Kava's death and the aftermath.

The first man's police statement was read in court as he was overseas and could not be located.

Nathan Tuvae said it was his first time on a boat and as he got onboard via the duckboard he noticed the BBQ and the small space.

He noted it was only about 1m wide.

"I could see that it was easy for someone to fall into the water," he said.

Tuvae told police that there was a safety briefing about 10 minutes after the boat set off into the harbour - but he was not sure if everyone heard the details, or listened.

After the briefing, the party started up.

Tuvae said everyone was "mingling" and some people were dancing.

Some people were intoxicated - but not overly.

"People were having fun," he said.

Tuvae was inside the boat looking out to the duckboard when Kava fell.

"I saw him lean against rail looking at my direction," he said.

"All of a sudden i heard someone yelling out to the captain to stop the boat ... Everyone was looking out the windows for Dave.

"I didn't take it seriously, didn't think anything like that could happen.'

Tuvae went to the duckboard to help look for Kava.

"I was holding onto the ramp and all of a sudden the ramp fell into water, I fell into the water as well," he recalled.

"I floated to surface noticed boat had gone ahead ... it was going pretty quick and it was pretty dark."

A relative jumped into the water and swam to Tuvae to see if he was okay.

The pair then swam towards the boat and a life ring was tossed out to them to help them back on board.

"I sat on top level in shock at what had just happened to me and Dave going missing," he told police.

He said the boat kept "circling around at sea" with crew using a spotlight on the water to try and find Kava.

Eventually they headed back to shore.

"It was a sombre mood back to shore," he said.

The boat was met by police and all on board were interviewed into the early hours of the morning.

A second partygoer was standing just inches from Kava when he fell.

Patrick Nauer had been down on the duckboard with another man when Kava climbed down.

Shortly before that, crew members had cleared the duckboard as there were too many people down there.

Seconds after the extras went back to the main part of the boat, Kava came down.

Earlier he had told people that he could not swim and he had been heard saying that "he hoped he didn't fall in".

Nauer said as Kava came towards the duckboard, he "stumbled".

Nauer helped him up.

He said Kava had been drinking but was not in a drunk state.

Kava laughed, then leaned back on the ramp to light a cigar.

Then, he fell.

"He moved back to spark his cigar and that's when he leaned back ..." he said.

"It just broke up."

The second witness said there was no signage near the duckboard to indicate how many people were allowed down there.

The crux of the case appears to be how the ramp Kava leaned on was secured, whether it had been secured adequately on the night of the party and whether there were enough safety measures in place to prevent excess people gathering on the duckboard.

The trial continues.

The prosecution is likely to close its case today and then Judge Sainsbury will hear from the defence.