BRISBANE (AFP) - Australia's third-largest city of Brisbane and other parts of Queensland state will enter a snap Covid-19 lockdown from Saturday (July 31) as the 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," Mr Miles said.

There were now seven cases of the Delta strain of the coronavirus in Queensland mainly linked to a school student, her family and a tutor, but the authorities were still trying to trace the source of the outbreak, Mr Miles 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.

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

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