Queensland has recorded nine new community cases of Covid-19, the biggest daily spike in almost a year.
Acting Premier Steven Miles said all new cases were linked to the state's new outbreak, which triggered a three-day snap lockdown yesterday afternoon.
Queensland chief health officer Jeannette Young said yesterday the Delta strain "could be anywhere" in southeast Queensland after a medical student, thought to be the index case, "travelled widely through Brisbane" while infectious.
She said the medical student was a tutor for a 17-year-old high school girl whose case was the first in the outbreak to be announced on Friday - and whose family members are also now infected.
Young said the medical student had been to a "lot of venues" and that there would be an "enormous number of exposure sites" all through Brisbane, as well as through the Sunshine Coast and further out.
There are now 18 cases in the cluster.
Authorities are scrambling to determine how the medical student contracted the virus in the first place.
Genome sequencing has matched the infection to two returned travellers who flew to Brisbane on June 29, according to ABC.
Young said health officials are working to find the missing link between the travellers and the medical student.
"I think that one of those two [returned traveller] cases has led to transmission to someone, who I don't know, who has then led to further transmission and eventually, I think the most likely scenario, is this medical student has acquired it," she said. "I don't know how."
Exposure sites include Royal Brisbane and Women's Hospital at the University of Queensland and at the Translational Research Institute at the PA, while the medical student also has a sibling who works in a hospital.
"Then we have the child, the youngest child in that family attends Ironside School, and one of the teachers there is already positive," added Young.
"So, we yesterday put all of these students and staff who attended the high school – the Indooroopilly State High School – into 14 days' quarantine. We will be doing the same today with all students and staff who have attended Ironside School."
Queensland Deputy Premier Steven Miles confirmed all the cases in the state's outbreak have contracted the Delta strain, which has been driving the outbreak in Sydney.
"We know from that experience that this Delta strain can spread very, very quickly. It is much more dangerous than the previous strains that we have been dealing with," he said.
"It means these new cases mean that there are now seven cases in the outbreak that we notified yesterday. We have seen from the experience in other states that the only way to beat the Delta strain is to move quickly, to be fast, and to be strong."
The Queensland lockdown applies to 11 local government areas (LGAs) in southeast Queensland from 4pm Saturday.
The LGAs are: Brisbane City, Moreton Bay Regional Council, Gold Coast, Ipswich, Lockyer Valley Regional Council, Logan City, Noosa Shire Council, Redland City, Scenic Rim Regional Council, Somerset Regional Council and Sunshine Coast Regional Council.
People can only leave home for four reasons: to obtain essential goods like groceries and medications – but only within 10km of their homes, for essential work or childcare, medical care and exercise.
Brisbane residents have been urged to monitor the Queensland Health website, with exposure sites expected to be added over the weekend.