The National Disaster Management Office (NDMO) authorised the MV Touaraken to sail from Port Vila to the island of Tanna with supplies of fuel and cargo.
But when the ferry docked at Tanna's Lenakel wharf, authorities found four passengers on board.
The Vanuatu Daily Post reported Tafea police detained the captain for 24 hours before he was allowed to sail back to Port Vila.
The four passengers have been isolated at Lenakel Hospital for 14 days.
