A boat company has to pay more than $93,000 in fines and reparations after a 60-year-old passenger's back was broken during a trip in the Bay of Islands.

Seafort Holdings Ltd and its sole director and shareholder, Richard Prentice, is being hit with the penalty after the woman suffered the back injury in 2014.

The passenger, Raewyn Russell, was sitting in the front row of seats in the company's vessel "Mack Attack" when the boat struck a large wave at speed and landed heavily.

Maritime NZ northern regional manager Neil Rowarth said that when the accident happened on 18 October 2014, Russell and her husband Craig Russell were sitting in the front row of seats.

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"As the vessel was nearing Cape Brett it struck a large wave at speed and landed heavily," Maritime New Zealand said in a release.

"Mrs Russell was thrown forward and on landing back in her seat heard a crack in her back and doubled over in pain.

"An ambulance was called to meet Mack Attack at Waitangi Wharf. Russell was taken to Bay of Islands Hospital and found to have a fracture in her eleventh thoracic vertebrae."

Seafort and Prentice were ordered to pay $32,630 in reparation to Russell and the company was fined another $55,000.

Prentice was fined $5500 for failing to take practical steps to help keep passengers safe.

The boat involved in the incident, Mack Attack, is a catamaran that can reach speeds of up to 100km/h.

It was being used for tourist excursions at the time of the incident. The trips mainly departed from Paihia.

Prentice and his company were fined following an investigation and prosecution by Maritime NZ.

They pleaded not guilty and a trial then began in September last year.

Judge Field found both Prentice and his company guilty last month of one charge each under the Maritime Transport Act.

The offence was that they "omitted to do an act that caused unnecessary danger or risk to any other person".

The judge found Seafort Holdings and Prentice had not advised the passengers of the known, heightened risk of back injuries, even more so for older people, sitting at the front of high-speed passenger vessels, Maritime New Zealand said.

"This injury is a permanent disability which has resulted in Mrs Russell giving up her career," Judge Field said.

Russell had worked as a manager and travelling salesperson.

Rowarth said the convictions sent an important message to masters and owners of passenger vessels: "You have responsibility for your passengers' safety. You must take practical steps to help keep your passengers safe."

Seafort Holdings was fined $90,000 in 2012 when a woman was left a paraplegic after a rough trip in the Bay of Islands.

The company was ordered to pay fines of $30,000 at the time over two charges connected to the incident.

The company also ran into issues in 2013 when Mack Attack was involved in a near-miss incident with a ferry.

The jet boat failed to give way to a ferry in Paihia and cut across the bow of the Bay Belle ferry.

Prentice and Seafort Holdings were found guilty in the Kaikohe District Court of operating a ship in a manner causing unnecessary danger or risk to another person or property. Prentice was also found guilty of failing to report the incident to Maritime NZ.

In a sentencing on Monday in the same court, Seafort Holdings was fined $18,000 and Prentice $2000 on each of his two charges.