"The ABF is aware a contracted service provider at the Melbourne Immigration Transit Accommodation (MITA) Broadmeadows Residential Precinct has tested positive for COVID-19," the spokesperson said.
"The individual does not have contact with detainees as part of their role and had last worked more than 12 days ago.
"The ABF and the contracted service provider have worked with Public Health to minimise transmission risk in accordance with infection control and public health procedures."
Earlier, an asylum seeker who is being held at Melbourne's Park Hotel told AAP residents were fearful the virus might spread.
Guards often shift detainees between the hotel and MITA, but the ABF officer told the asylum seeker that the guard hadn't visited the hotel.
"This guy came and ABF - and he was like saying one of the staff get COVID but he is not been here - he didn't come here (the hotel)," the asylum seeker told AAP.
"And people get very angry here."
The man said fellow asylum seekers detained at the hotel were worried they were being put at risk as guards there had not been wearing their face masks properly.
Refugee Action Coalition said they had been told another guard at MITA has also tested positive with at least nine other guards who were working on Saturday afternoon told to isolate.
"The federal and state governments have been warned so many times that it was just a matter of time," RAC spokesperson Ian Rintoul said in a statement.
"The federal government 's own literature says detention centres are a danger zone.
"The detention is a particular worry, as many detainees have underlying medical conditions that make them more vulnerable to serious illness.
"The people in immigration detention should be released. It is a disgrace that they have not been released before this, but it's time for the government to act."
Speaking to the media on Sunday morning, Victorian COVID-19 Commander Jeroen Weimar confirmed one guard had tested positive for COVID-19, but said he was unaware of any further cases in the facility.
"The individual was a security guard who worked on the outside-facing parts of the premises, so (there was) no contact within the facility or any of the staff who worked within the facility.
"We have had no positive test come out of that facility at the moment, with is good news and we will continue to maintain the wider support around there."
The ABF spokesperson said they have started rolling out coronavirus vaccines "for all detainees who wish to receive them" around the country.