The fourth case, also linked to an existing case, is understood to be a student at Staines Memorial College in Redbank Plains.
That school will be shut for the next few days while cleaning is carried out, and the students and their families have been alerted to the case.
Queensland Chief Health Officer Dr Jeannette Young on Sunday said authorities now believe they have identified at least one link between the two clusters at the Queensland Youth Detention Centre at Wacol and the Queensland Corrective Services Academy.
The two outbreaks have been assumed to be one cluster, however contact tracers have been searching for the link between them.
“One of the cases who was a close contact of a worker at the Brisbane Youth Detention Centre is a friend of one of the partners of one of the workers at the academy,” Dr Young said.
“There are a lot of links between the people who work at the Brisbane Youth Detention Centre and the people who have been attending the academy, as you would expect.”
Dr Young said the cluster was “growing” but she believed it could still be contained if people adhered to social-distancing guidelines and wore masks in places where they were not able to socially distance.
She also said they believe that “on the balance of probabilities”, the now-joined clusters are linked to the earlier Logan cluster, sparked after three women went to Melbourne and returned to Queensland without declaring where they had been.
The new cases bring Queensland’s active cases to 28 out of a total of 1121 since the pandemic began.
It comes as the Queensland government launches a mental-health wellbeing push for people struggling under pandemic restrictions and the general anxiety of a global pandemic.
The first phase of the “Dear Mind” campaign was launched in January just before the start of the pandemic, and Premier Annastacia Palaszczuk said it would now be extended for those in need during stressful times.
“This pandemic is taking a toll on everyone, and everyone will be impacted in some way, whether that’s not seeing friends and family as much as they used to or, with restrictions in place, not being able to go and see loved one in aged-care facilities,” Ms Palaszczuk said.
“We have had a lot of change in our community, so it is absolutely crucial that we talk to our loved ones, our friends and our family, and just ask them are they going OK.”
A total of 10,558 tests were carried out in Queensland in the 24 hours to Sunday morning, as people with symptoms were urged to get checked, especially in south-east Queensland.
The state is preparing to extend private and public gathering restrictions from Monday to help contain the spread of the virus.
After a confirmed case travelled to Toowoomba, from 8am on Monday, gatherings will be limited to 10 people in private homes and public areas in the Western Downs, South Burnett, Cherbourg, Toowoomba, Goondiwindi, and the Southern Downs council areas.
Those restrictions already apply in the Greater Brisbane, Ipswich and Gold Coast areas, but do not apply to public venues that already have a COVIDSafe plan in place.
Stuart Layt covers health, science and technology for the Brisbane Times. He was formerly the Queensland political reporter for AAP.