The man in his 30s returned to his home in Wollert, in Melbourne's north, on Tuesday last week and developed symptoms on Saturday.

He has since returned a positive result after testing on Monday, with a re-test on Tuesday confirming his infection is active.

The case has sparked a full public health response as the two states work to discover the source of infection.

"This case is an important reminder that we are still in the grip of a global pandemic," Victorian Health Minister Martin Foley told reporters.

Victoria's Chief Health Officer Brett Sutton thinks the man did not acquire his infection after returning to Victoria.

"It's absolutely the case that he's picked it up in quarantine in South Australia," he said.

Contact tracers have now finished their initial interview with the infected man and all three of his household contacts returned negative test results.

"It's an early encouraging sign," Professor Sutton said.

Four "tier one" exposure sites have been identified including an Altona North office, a CBD Indian restaurant and bar, Woolworths in Epping and a spices and groceries shop. Anyone who visited the sites at specific times must be tested and isolate for 14 days.

Also on Tuesday, staff at Australia's largest pathology lab software provider had to evacuate their Melbourne office due to the case.

Citadel Health on Collins Street shut down after discovering a staff member was a close contact of the Melbourne case.

More than a dozen staff members evacuated, but the colleague has tested negative.

SA health authorities say they do not believe the case represents a risk of community infection in their state.

"I don't think he was infectious here in SA given the timing of his symptoms," the state's Chief Health Officer Professor Nicola Spurrier told the media in Adelaide.

Prof Spurrier believes it unlikely the man was infectious while in quarantine in South Australia or on the flight back to Melbourne, and predicts his infectious period would have begun last Thursday.

But she conceded it was probable that transmission had occurred in hotel quarantine.

Authorities are examining whether the man caught the virus before he entered SA and simply had an unusually long incubation period, or whether he caught it from a person with the virus in the room next door at the Playford Hotel.

The positive Melbourne case returned from overseas on April 19, flying from India via the Maldives and Singapore before Australia's flight ban came into effect.

He recorded negative tests on day one, five nine and 13 in hotel quarantine but was staying in a room adjacent to a fellow returned traveller who tested positive on day nine of their two-week stay.

Staff reviewed CCTV and did not find any infection control breaches, but are now re-checking the footage.

There are currently new guests staying on the floor where the positive cases were.

Prof Sutton said it is likely the case will be classified as an interstate-acquired infection, continuing Victoria's 73-day streak without a local case.

Meanwhile, Victoria Police officers are in isolation after rescuing a man believed to have jumped from an oil tanker in Melbourne.

The 31-year-old Georgian national was rescued in Port Phillip Bay on Sunday night, about five kilometres from where the international tanker had docked.

The police air wing found the man clinging to a navigational marker, and the water police officers who rescued the man have gone into isolation as a precaution.

Australian Associated Press