Brisbane International Airport is now a "venue of concern" after a passenger, who had travelled from Papua New Guinea and mingled with passengers bound for New Zealand, tested positive to Covid-19.
Almost 400 passengers who flew to New Zealand from Brisbane have potentially been exposed to the virus.
The infected man travelled from a "red zone" country but was inadvertently allowed to cross into the "green zone" area at Brisbane Airport where passengers were waiting to travel to New Zealand.
The infected man and a companion unintentionally spent two hours in the green zone on Thursday morning before they were escorted to their rightful area.
However, in that period, 390 passengers boarded three flights bound for New Zealand.
People arriving in New Zealand from Brisbane have been asked to monitor their health for the next 14 days.
Queensland chief health officer Jeannette Young has declared the international terminal a venue of concern.
"Anyone who was in the terminal between 9.45am and midday on Thursday, 29 April 2021, should monitor their symptoms and get tested immediately if they feel unwell," she said.
"Once the mistake was identified, the two passengers were tested for Covid-19.
"One man's initial test was equivocal, meaning it was neither positive nor negative. Further testing revealed he is positive. The other passenger is negative."
Prior to entry into Queensland, both passengers reportedly tested negative for Covid-19. Both passengers were tested again in Brisbane yesterday, with one passenger returning a negative result.
Dr Young said New Zealand authorities had been alerted to the error and the risk of Queenslanders becoming infected was low.
The three flights affected are Air New Zealand NZ202 from Brisbane to Christchurch, Air New Zealand NZ146 from Brisbane to Auckland, and Qantas QF135 from Brisbane to Christchurch.
"Staff who came into contact with this case have been placed into quarantine," she said.
"We have advised the NZ Ministry of Health of the latest results, and they are taking their own protection measures."
The two passengers who arrived on Thursday morning into the red zone were let into the area designated for New Zealand arrivals due to "human error", Brisbane Airport Corporation confirmed.
The pair dined at Hudson Cafe for about 90 minutes before using the toilets and visiting a second retail outlet.
They were eventually found by airport staff and returned to the red zone. Three "green" flights departed within the two-hour period to New Zealand.
Young earlier said the two passengers posed a low risk to others.
"While at the airport, they wore masks and socially distanced and neither has reported symptoms," she said.
"They were in the wrong zone through no fault of their own, and we appreciate their patience and co-operation while we rule them out as cases.
"We're also grateful for the prompt action by Brisbane Airport staff once the mistake was identified."
Meanwhile, 246 people have been told to urgently take a coronavirus test in Melbourne after a "strong and unexpected" surge in viral fragments in wastewater in the city's western and northwestern suburbs.
"This additional action is being taken due to the strength of the wastewater detection and because a known positive Covid-19 case, from flight QF778, has been in Victoria in the past 14 days," the Victorian health department said in a statement.
The 246 people who have been contacted today include four primary close contacts of that case and 242 recently returned red and orange zone travel permit holders.
"All of these primary close contacts have recently been tested and have returned negative results. All of the 246 people are being asked to test again out of an abundance of caution," the department said.