Sailings to and from Melbourne and Devonport were cancelled on Friday night when the positive case emerged among a TT-Line employee.

The contractor worked below deck on both Spirit of Tasmania vessels on Monday and Tuesday while infectious.

"We've identified each of the people ... that were involved in that exposure," Public Health acting director Dr Scott McKeown told reporters on Saturday.

"There was a small group of people, 17, who were exposed across both vessels. No other workers had exposure to this case at all."

Eleven are in Tasmania and all have since returned negative results from rapid tests, while two of six in Victoria have also tested negative.

Another 67 crew members are also undergoing precautionary testing port-side in Devonport.

"The risk to passengers remains very low and as a result they've been able to disembark the ship if they wish," Premier Peter Gutwein said.

The cancelled sailings disrupted the travel plans of about 150 passengers in Melbourne as well as 40 to 50 in Devonport, with almost all opting to stay in their cabins overnight.

The Devonport ferry is poised to set sail on Saturday, while the Melbourne vessel can follow suit once the four remaining Victoria-based primary close contacts return negative results.

"We apologise to any of our passengers that have been disrupted by the actions that were taken last night," TT-Line Chief Executive Officer Bernard Dwyer said.

"But to keep Tasmania safe and Tasmania free of COVID as long as we possibly can, these actions were extremely necessary."

All passengers will be offered full refunds or have their bookings rescheduled free of charge.

Mr Gutwein said the scare demonstrated Tasmania's COVID-safe systems were working to keep the Delta variant from creeping in from Victoria before the state hits higher vaccination targets.

"If Delta gets loose here, people will die. I'm so very pleased that we're in the position we are today," he said.

Australian Associated Press