Australia's Queensland state has extended a Covid-19 lockdown in Brisbane, while soldiers have begun patrolling Sydney to enforce stay-at-home rules as Australia struggles to stop the highly contagious Delta variant of the coronavirus from spreading.

Queensland says it has detected 13 new locally acquired Covid-19 cases in the past 24 hours — the biggest one-day rise the state has recorded in a year. The lockdown of Brisbane, Australia's third-biggest city, was due to end on Tuesday but will now stay in place until late on Sunday.

"It's starting to become clear that the initial lockdown will be insufficient for the outbreak," Queensland state Deputy Premier Steven Miles told reporters in Brisbane.

The rising new case numbers in two of the country's biggest cities comes as disquiet grows on how the government of Prime Minister Scott Morrison is handling the pandemic.

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