South Australia will keep its border open with Victoria but has imposed new testing requirements for people travelling from Greater Melbourne.
Those people will be required to have a coronavirus test on days one, five and 12 and will be asked to isolate until they receive a negative result from the first test.
The same will apply to anyone from Greater Melbourne who arrived in SA since January 28.
However, any who attended at the Grand Hyatt Hotel in Melbourne since then will be placed in hotel quarantine for 14 days.
Other people who visited a range of locations of concern across Melbourne will also be asked to self-isolate.
It's believed up to 8000 people have travelled to SA from Victoria in the past week.
That's expected to result in a spike in testing on Thursday, with extra capacity brought on to cater for the demand.
Premier Steven Marshall said the discovery of a virus case in a hotel quarantine guard in Melbourne was concerning and it was still early days.
"We do have to be alert, we don't have to be alarmed," he said.
"We certainly have been able to deal very effectively with these types of situations before and I expect that it's going to continue in the future."
Acting Chief Public Health Officer Emily Kirkpatrick said officials were receiving good information from their Victorian counterparts.
"We've had excellent information come across last night from Victoria, and then again this morning," she said.
"We feel very confident that they have moved very quickly to undertake contact tracing and testing, putting close contracts into quarantine."
Police Commissioner Grant Stevens said no serious consideration was given to closing the border with Victoria at this stage.
"Obviously it was something that is in your mind in terms of what actions may be required based on the information we were provided, but there was no movement towards that," he said.
"We believe that what we're proposing going forward for Victoria is a reasonable step to protect South Australians from COVID-19 without inhibiting that movement between states unnecessarily."
SA's transition committee will meet again on Friday to discuss the Victorian situation.
SA Health reported no new virus cases on Thursday.
There are two people with active infections in hotel quarantine, both returned travellers.