South Australia has slammed shut its border for any travellers from Greater Melbourne as a COVID-19 cluster of cases there continues to grow.
Health officials have also released a report which suggests aerosol transmission may have caused an infection in a Victorian man in Adelaide hotel quarantine, which ultimately sparked the Melbourne outbreak.
The report found no obvious breaches in SA's hotel quarantine operations but said the man was staying in a room near where another confirmed case emerged. Both rooms were at the end of a corridor, reducing air flow.
It has also pointed to the times when the doors of both rooms were opened, so their occupants could collect meals.
"There was no high-risk single event or high-risk breach in infection prevention and control practices identified during this investigation," the report said.
"Therefore, it is highly likely the close timing of doors opening and closing between adjacent rooms was responsible given the clear role of aerosol transmission of this virus."
Among the report's recommendations are a move to keep room deliveries to a minimum, more education for guests on the possible transmission of the virus, continued review of hotel ventilation systems, and a requirement for those in quarantine to have a test on day 17, three days after being released.
It also suggested higher separation among guests considered to be at higher risk of incubating an infection.
"These may seem like small things that we need to do in terms of tweaking our medi-hotel system but this is part of a continuous quality improvement," Chief Public Health Officer Nicola Spurrier said.
"We absolutely want to minimise the times that this happens. It's had a huge impact on Victoria and it's now having an impact on us."
With 15 confirmed infections in Melbourne, SA will only allow essential workers and returning South Australians to cross the border, but they must get three virus tests and isolate for 14 days.
Anybody from Greater Melbourne who travelled to SA since May 20 must also get tested and isolate until they receive a negative result.
"Our thoughts are with everybody in Melbourne. Any outbreak is something we need to take extraordinarily serious," Premier Steven Marshall said.
Professor Spurrier said the information coming from Victoria was very concerning considering the large number of exposure sites across Melbourne.
She said three South Australians had also been identified as close contacts of a confirmed case in someone who attended the Port Adelaide and Collingwood AFL game at the MCG last weekend.
They have been asked to isolate, with one of those three still in Melbourne.
Another 464 South Australians have been identified as being at the ground and are also being asked to self-isolate for two weeks and get tested three times.
SA reported two new virus infections on Wednesday, both returned travellers in hotel quarantine.
The state has three active infections, all in hotel quarantine.