Victory II should have been carrying no more than five tonnes of fish, but when it sank near Kaikoura in 2017 it had at least 28 tonnes on board.
Maritime New Zealand central region compliance manager Michael-Paul Abbott said it was the equivalent of putting 20 medium-sized cars on a small fishing boat.
The Wellington fishing company Nino's Ltd, its sole director Antonio Basile and the vessel's master Shane McCauley were fined $449,500 and the company has been ordered to pay $64,800 in reparations to the crew for unpaid wages.
"Overloading sinks ships. The boat sank in only two minutes. The four men on board were very fortunate to be rescued. This trip could have ended in tragedy and the lives of the crew were unnecessarily put in danger."
The boat fished between June 8 and 10 off Kaikoura, when the conditions were good.
In spite of the easy sailing conditions, during its return voyage to Wellington the back deck of the overloaded vessel flooded and it began to sink.
At 11.27am the master radioed a distress call and the crew launched a life raft; however, they were unable to put on life jackets because the vessel sank too quickly.
Helicopters were mobilised but the four men were rescued by a local fisherman, who had launched his boat on hearing the distress call and took them to shore.
Maritime NZ investigated the sinking, and prosecuted the company, its director, and the master.
They pleaded guilty to three charges under the Health and Safety at Work Act, and the company also pleaded guilty to two further charges under the Maritime Transport Act.
They were sentenced in the Wellington District Court yesterday.
Basile and McCauley each pleaded guilty to one charge of exposing people to risk of death or serious injury by failing in their duty to ensure, as far as reasonably practicable, people's health and safety.
Neither of the pair were aware the vessel's capacity was five tonnes, and only Basile had seen the vessel's "stability book", which sets the limit on loading and includes other safety information.
As well as this, the investigation also revealed Nino's had stopped paying wages to two of Victory II's crew after the sinking. Employers are required to pay seafarers' wages for two months following a sinking or until they have new employment.
Nino's pleaded guilty to not paying two of its workers after the sinking - one of whom was owed 47 days' pay.
Basile said they did not want to comment on yesterday's outcome but they would be back to business and enjoying the sunshine.
