Queensland has recorded 13 new community cases of Covid-19, all linked to the existing outbreak that prompted a lockdown in the state's southeast.

Deputy Premier Steven Miles said the growth of the cluster to a total of 31 cases required a 5 day extension of the current restrictions.

"It's starting to become clear that the initial lockdown will be insufficient for the outbreak," he said.

"So we're advising southeast Queenslanders in the 11 LGAs that the lockdown will be extended until 4pm on Sunday. That will make it an 8-day lockdown."

Queensland's health minister Yvette D'ath urged residents to adapt to the new threat of the Delta strain.

"This lockdown is harder for a good reason. We're trying to save lives," she said. "The rules are stricter this time, so please don't rely on what you've known in the past."

Virus 'could be anywhere' around Brisbane

Queensland's chief health officer Jeannette Young said on Saturday 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.

Authorities are still working to determine how the medical student contracted the virus before passing it on to a 17-year-old high school girl whose case was the first in the outbreak to be announced.

Genome sequencing matched the infection to two returned travellers who flew to Brisbane on June 29, but Young said health officials have not found the missing link between the travellers and the medical student.

"I genuinely don't know how it's got from those two people who came in to Queensland on June 29 and now we've seen this outbreak," she said today.

"I don't know how it's got from those two original people to this household of five who are the first people that I've found in the cluster."

Queensland chief health officer Jeannette Young said health officials have not found the missing link in the state's new outbreak. Photo / Getty Images

An Australian infectious disease expert has warned the Queensland outbreak has the makings of a "very significant event", similar to the situation being faced in Greater Sydney.

Director of Infectious Diseases at Mater Health, Dr Paul Griffin, told Weekend Today that Queensland had been fortunate to avoid widespread outbreaks of the Delta variant so far, but warned this situation could easily get out of control.

"This one is very different. This one really does have the makings of something significant," he said.

"There's a lot of concerning elements about this, including where these people have been, how many exposure sites there are, how many people are already infected as a result. So I do think we need a comprehensive and swift range of mitigation strategies which is what we've seen."

Health workers at a pop up Covid-19 testing clinic at in Brisbane. Photo / Getty Images

Thousands of students, staff and family members across five major Brisbane schools are in lockdown, and dozens of exposure sites including the University of Queensland and Westfield Indooroopilly have been listed.

Griffin said the snap lockdown announcement was the right approach, warning delayed action could result in an outbreak on scale with the one currently ravaging Sydney.

"Hopefully the lockdown allows contact tracers to get on top of this," he said.

"If [lockdown] doesn't all go according to plan, we certainly could have sustained community transmission arising from this event."

The Queensland lockdown applies to 11 local government areas (LGAs) in the southeast and affect approximately 3.8 million residents out of the state's population of 5.1 million.

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 grow.

The NZ Ministry of Health is urging people who have recently returned from Queensland to check whether they visited any of the state's locations of interest.

Director general of health Dr Ashley Bloomfield said anyone who had visited the relevant locations should immediately isolate and call Healthline on 0800 358 5453.