Tasmania's border with Victoria will be shut from midnight on Friday in response to a growing coronavirus cluster in Melbourne.
Premier Peter Gutwein says the length of the closure is expected to follow Victoria's snap five-day lockdown which begins at 11.59pm.
"The message is very clear, don't come to Tasmania," Mr Gutwein said.
"I would hope this is a temporary measure. We will be cognisant of what occurs in Victoria over the next five days."
No one from Victoria is allowed to travel to Tasmania unless they are granted special permission as an essential worker.
Tasmanians returning to the island state will still be allowed in but they must quarantine for 14 days either at home or a government-run hotel.
Melbourne's COVID-19 outbreak, connected to the Holiday Inn at Tullamarine Airport, has grown to 13 cases.
They include a female assistant manager and four close contacts of people who earlier tested positive for the virus.
"This is a very serious threat, nationally really. Tullamarine is a major transport hub," Tasmanian Public Health Director Mark Veitch said.
People in Tasmania who were at Terminal 4 of Tullamarine Airport on Tuesday must self-isolate and call public health immediately.
Anyone at the airport on Sunday and Monday should monitor for symptoms and get tested if any develop.
"We're on the cusp of a weekend, the last thing we want ... is someone from Terminal 4 going out and infecting hundreds at a pub or club," Mr Gutwein said.
Some 800 to 1000 people in Tasmania who went through the terminal at risky times have been contacted by the state's health department.