A woman who contracted the mutant UK strain of Covid-19 and flew into Brisbane from Melbourne after clearing hotel quarantine has since tested positive to the virus again, prompting an urgent public health response.
Under the previous national protocols, the woman was allowed to leave after 10 days in hotel quarantine, and did not need to return a negative test before doing so.
But because authorities are so concerned about this new strain, those rules have now been changed.
What do we know about this case?
Queensland's Chief Health Officer Jeannette Young said the woman flew into Melbourne from the UK on 26 December and tested positive a day later.
She spent 10 days in hotel quarantine, "cleared all her symptoms and was allowed to leave Victoria", Young said.
The woman caught flight JQ570 from Melbourne to Brisbane, arriving at 11:00pm (local time) on 5 January. She then travelled to her parents' house in Maleny, on the Sunshine Coast.
Young said Victorian authorities alerted their Queensland counterparts the woman had tested positive to the UK variant. She was retested in Queensland yesterday and found to still be positive.
Authorities said she posed a very low risk, and they were contact tracing out of an abundance of caution because of this new strain.
So why was she allowed to leave hotel quarantine?
Until recently, people who tested positive to Covid-19 while in hotel quarantine could leave once they had no symptoms for three days, and at least 10 days had passed since they delivered the positive test.
That's in contrast to the mandatory 14 days - based on the life cycle of the virus - in place for travellers who do not contract the virus.
That was under guidelines from the Communicable Disease Network Australia, which Victoria and other states and territories had followed.
Victorian Health Minister Martin Foley said "the person wasn't infectious" when she left the Victorian quarantine system.
She was cleared on day 10 of hotel quarantine after showing no symptoms for three days.
"That was totally in line with both Australian national and international standards and protocols," Foley said.
Under those old guidelines, which were in place since March 21, travellers with a mild case of Covid-19 did not need to return a negative test before leaving hotel quarantine.
One of the reasons for that is people who have contracted the virus can still test positive to Covid-19 after they have passed their infectious period, in a process known as "shedding".
Why are authorities worried?
The woman who left Melbourne contracted the same UK strain as the Brisbane hotel quarantine cleaner - whose positive result has sent Greater Brisbane into a three-day lockdown and seen borders slammed shut across the country.
Research estimates the UK variation could be up to 70 percent more infectious than other strains, and authorities fear transmission could be hard to control if it gets into the community.
"We have to assume that this strain will become the dominant strain and it is important to keep re-assessing our settings, keep staying vigilant," New South Wales Premier Gladys Berejiklian said.
Queensland's Dr Young said the risk of the woman passing on the virus was very low, and "with a normal variant we would not be as concerned".
The UK strain is just one of a number of new variants of Covid-19 that appears to be more transmissible and therefore more concerning to authorities.
"We are seeing the emergence worldwide of a number of strains and, with travel, they are no longer the UK strain or the South African strain," New South Wales Chief Health Officer Kerry Chant said.
"It is important that we live in a global world and so all returning travellers are at increasing risk of having one of these mutations."
Chant did clarify that there was at this stage no evidence the new strain behaved any differently to the dominant strain when it came to being cleared of the virus, but authorities were taking no chances.
What rules are changing as these new strains emerge?
The guidelines from the Communicable Disease Network Australia have been updated to advise anyone who tests positive for the mutant UK strain to quarantine for the full 14 days from the onset of symptoms.
In Victoria, those changes came into effect on 6 January - a day after the woman discussed today flew into Queensland.
Chant said the advice now included giving people a PCR test at the end of their isolation period to show they were no longer infectious.
"Now, it is important to note that some people still can have remnants of the virus for a long time, so we will use an expert panel to ensure we are not releasing cases that are infectious and that will require more intensive testing if anyone still remains PCR-positive," she said.
Chant said out of an "abundance of precaution", NSW authorities had also updated a list of exposure sites that a person who was previously cleared of a mutant strain had visited.
"We're taking a very precautionary role as we learn about this disease," she said.
Yesterday's National Cabinet meeting saw a number of other rules tightened to guard Australia against the strains.
International passenger caps have been reduced, masks are now mandatory in airports and on planes, tests will be compulsory before and after international flights and there will be daily tests for hotel quarantine workers.
The increased testing for quarantine staff was already in place in Victoria, which has dramatically overhauled its system in the wake of the state's deadly second wave.
NSW Health Minister Brad Hazzard said daily testing being introduced in his state was "an important step forward, particularly to manage the increased issues related to these new variants".