Troops were set to hit the streets of Sydney on Monday to enforce its prolonged lockdown, as stay-at-home orders in Australia's third-largest city Brisbane were extended to curb a worsening outbreak.

About 300 Australian Defence Force personnel will be deployed in Sydney after New South Wales state police requested military help to enforce Covid-19 rules.

Authorities have been struggling to stop the spread of the highly infectious Delta variant in Sydney -- and ensure that residents follow containment rules -- with more than 3,600 cases recorded since mid-June.

"Police officers will be assisted by ADF personnel as they deliver food parcels, conduct welfare door-knocks and go through compliance checks of stay-at-home and self-isolation orders," NSW Police Commissioner Mick Fuller said Saturday.

More than five million people in Australia's biggest city and surrounding areas are entering their sixth week of a lockdown set to run until the end of August.

Residents are only allowed to leave their homes for exercise, essential work, medical reasons, and to shop for necessities such as food.

But compliance has been patchy and police have increasingly been doling out fines to those violating the restrictions.

The defence force said the latest deployment was in addition to the 250 military personnel already working at hotels and airports in New South Wales.

Meanwhile, millions of people in Brisbane and several surrounding regions will remain under lockdown until Sunday after an "escalating" outbreak there grew to 29 cases.

Those stay-at-home orders had been scheduled to lift on Tuesday.

"That will make it an 8-day lockdown. And we desperately hope that that will be sufficient for our contact tracers to get into home quarantine absolutely anyone who could have been exposed to the Delta strain," acting Queensland state premier Steven Miles said.

The outbreak was linked to a Brisbane school student, with pupils and teachers at several schools subsequently placed into isolation.

With about 14 percent of Australia's 25 million people fully vaccinated, authorities are still relying on lockdowns to slow the spread of the virus.

Prime Minister Scott Morrison has outlined a long road out of restrictions -- setting a target of 80 percent of the eligible population to be fully vaccinated before borders are reopened and lockdowns eliminated.

Australia has recorded more than 34,000 cases and 925 deaths so far during the pandemic.