Troops hit Sydney's streets to help enforce its prolonged lockdown while 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 to the country's largest city 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.

With thousands of close contacts of Covid cases told to test and stay at home for 14 days, police said they lacked the manpower to make sure everyone was complying.

Troops are expected to help police deliver food parcels, conduct "welfare door-knocks" and check people are complying with self-isolation orders.

"I want to stress up front again that we will be under control of the NSW police," said Brigadier Mick Garraway.

"We are not a law enforcement agency and we will do tasks that are supportive in nature."

More than five million people in Sydney 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.

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

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

"That will make it an eight-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.

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

Defence Minister Peter Dutton's sons attend one of the schools hit, so he is among those being forced to quarantine at home for 14 days.

"Having had Covid and being fully vaccinated, I have also tested negative this morning," he said in a statement.

With about 15% 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% 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.

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