Experts say there's only an "extremely low" chance the Melbourne COVID-19 outbreak has spread here, but closing the bubble is still the right thing to do.
Though there are just nine cases in the city of 5 million people, COVID-19 Response Minister Chris Hipkins on Tuesday said there are "several unknowns" about where it originated, so put a 72-hour pause on travel between the state of Victoria and here.
University of Auckland disease modeller Shaun Hendy told Newshub it was the right call, "given that the Victorian authorities haven't yet got to the bottom of this cluster".
"Although there's a presumed genomic link to a case that came in from South Australia, the precise chain of transmission is not understood at the moment. That does mean there could be other cases out there in the community."
There is a genomic link to an overseas traveller who caught COVID-19 during a stay in managed isolation in Adelaide, South Australia, before returning home to Melbourne, but no epidemiological link - that is, no known chain of transmission between them and the nine people who have tested positive.
"It is reassuring in a sense that we can trace it back to the origin, [but] it is suggesting there are other cases out there," said Dr Hendy.
University of Otago epidemiologist Michael Baker told The AM Show on Wednesday it's likely to be a "modest-sized outbreak" based on the gaps between the traveller, who arrived in Melbourne on May 4 and tested positive a week later, and the new cases.
"We don't know how big it will get. It's always a little worrying when you don't have a source for an outbreak - you don't know if you're looking at the tip of the iceberg, if there's a lot of cases you can't see."
Dr Baker said it was possible an infected person travelled to New Zealand in the meantime, but extremely unlikely "because the number of infected people in Melbourne is also extremely low".
"Well under 1 percent, probably several more noughts in there... There is that big proviso of course that any New Zealanders who have been in that area in the last two weeks need to look at that list of places of interest. If they're on that list, they need to talk to Healthline here. But I think it's still a very low-risk event."
Melbourne has quashed much bigger outbreaks than this one. There are fresh concerns Wednesday morning after it emerged one of the positive cases went to an AFL match at the MCG on Sunday, with 23,000 other fans.
Dr Hendy said whether the bubble can reopen in two-and-a-half days' time depends on how quickly contact tracers can track down everyone who might have been exposed to the virus, and a surge in testing.
"If the contact tracing and the surge testing doesn't turn up new cases, I think we can relax a little bit. However it may be that they find themselves in the situation we were in last August - where there was a big cluster behind things, in which case it might be some time before the travel bubble can resume."
"We've had some outbreaks, obviously in Perth as well and Sydney," said Dr Baker. "We suspended travel briefly then we got back to business as usual after that... Remember we've got the seven other jurisdictions in Australia that we can have flights from or travel from... much travel with Australia will just carry on regardless."