Underwear that had been flushed down an Interislander ferry toilet caused a family to get covered in urine when "unusually high pressure" built up in the system.

Sean Scanlon was travelling between Wellington and Picton with wife Lucy, 8-year-old son Jack and dog Cooper on Monday morning when the incident left them wet and smelling of the bodily fluid, according to Westport News.

While waiting to drive off the ferry when it arrived in Wellington, the family opened their car windows only to be splattered by a "shower of liquid and an immediate odour" from some drums and pipes being worked on by Interislander staff.

The liquid – urine from the ferry's toilet system – sprayed the family's car, hitting Scanlon and his son through the open windows and dripping down the inside of the doors.

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"It was a wee mistake," Scanlon told the newspaper.

While he could see the funny side, Scanlon recognised the seriousness of the incident and was immediately concerned for the workers, one of whom was "looking absolutely horrified, totally drenched in pee" while the other ran around trying to fix the problem.

The family had to disembark the ferry with the other vehicles. Phone calls to the Interislander on Monday and Tuesday yielded no response until it was contacted by Westport News, when it promised to review the call centre's lack of response.

Scott Hammond/Stuff Sean Scanlon and family were sprayed with human pee while they were in their car waiting to disembark the Interislander ferry.

Interislander executive general manager Walter Rushbrook told Stuff it had since apologised to the Scanlons and arranged compensation. It also apologised for its call centre not escalating the matter appropriately.

"Staff were working to clear a blockage in the sewage system which turned out to be caused by a pair of underpants flushed down an on board toilet."

An "unusually high pressure build up" resulted in the system spraying urine onto the vehicle deck.

More than 1000 people were sailing on the ship at the time, due to it being peak season, which meant there was only a short amount of time between sailings to attend urgent maintenance issues.

That was why workers were trying to fix the issue as passengers were exiting the ship.

"Our maintenance staff wear standard protective gear and have preventative immunisation shots," Rushbrook said.

Signage in the loos advised passengers to only flush the supplied toilet paper.

The company would review its processes and procedures following the incident, he said.