Australia's third-largest city of Brisbane and other parts of Queensland state will enter a snap Covid-19 lockdown from Saturday as authorities race to contain an emerging outbreak of the Delta strain.

Millions of residents in the city and several other areas will be placed under stay-at-home orders from Saturday afternoon for three days, state Deputy Premier Steven Miles said.

"The only way to beat the Delta strain is to move quickly, to be fast and to be strong," Miles said.

Six new cases were reported Saturday in a cluster of the Delta variant initially linked to a school student, resulting in pupils and teachers at two schools being placed into isolation.

Genome sequencing had connected the cluster to returned overseas travellers in hotel quarantine but the exact source of transmission remained unclear, Chief Health Officer Jeannette Young said.

In the "strictest lockdown" the city has enforced, residents will only be allowed to leave their homes for essential reasons, including buying groceries and exercising.

Authorities are still trying to trace the source of the outbreak in Brisbane, Australia's third-largest city
Authorities are still trying to trace the source of the outbreak in Brisbane, Australia's third-largest city AFP / Patrick HAMILTON

Brisbane's snap lockdown comes as Australia's largest city of Sydney and its surroundings completed a fifth week of lockdown, with authorities struggling to stop the spread of a Delta variant outbreak there.

"We cannot afford to be complacent just because we have done so well so far. We all we have to comply with these restrictions," Miles said.

Sydney recorded 210 new local cases on Saturday, slightly down from the record number reached earlier in the week.

Police were out in force around the city, attempting to prevent anti-lockdown protesters from gathering after thousands poured through the streets and sparked violent clashes with officers last week.

With close to just 14 percent of the population fully vaccinated, authorities around the country continue to rely on lockdowns to reduce people's movements and slow the spread of the virus.

On Friday, the country's Prime Minister Scott Morrison outlined a long road out of restrictions -- setting a target of 80 percent of the population to be fully vaccinated before the government would reopen borders and end lockdowns.

Coronavirus