A woman runs past the Opera House in Sydney on Saturday. AFP

Australia's largest city Sydney and some surrounding areas will enter a hard two-week COVID-19 lockdown on Saturday, authorities struggle to control a fast-spreading outbreak of the highly infectious Delta variant that has grown to 80 cases.

Sydney's usually bustling harbourside restaurants were shuttered and city streets nearly deserted as more than a million people in downtown Sydney and eastern suburbs were already under lockdown due to the outbreak, but health authorities said they needed to expand the curbs after more infections were recorded, with exposure sites increasing beyond the initial areas of concern, according to Reuters.

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The flare-up was a shock for a city that had returned to relative normality after months with very few local cases.

More than five million people across Sydney will be subject to movement restrictions from Saturday evening.

A food delivery worker rides in the central business district of Sydney on Saturday. AFP

"Even though we don't want to impose burdens unless we absolutely have to, unfortunately this is a situation where we have to," said New South Wales state Premier Gladys Berejiklian.

Australia has been more successful in managing the pandemic than many other advanced economies through swift border closures, social distancing rules and high compliance, reporting just over 30,400 cases and 910 COVID-19 deaths.

But the country has struggled with its vaccination rollout, and states have been plagued in recent months by small outbreaks. These have been contained through speedy contact tracing, isolation of thousands of people at a time or snap hard lockdowns.

A couple ride along the harbour in Sydney on Saturday. AFP

Saturday's lockdown in New South Wales will also include the regions of Blue Mountains, Central Coast and Wollongong, which surround Sydney.

Under the rules in place through July 9, people can leave home for essential work, medical care, education or shopping. The rest of the state will have limits on public gatherings and masks will be obligatory indoors.

"There was no point doing it for three days or five days because it wouldn't have done the job," Berejiklian told a news briefing.

A general view shows the usually-busy tourist area “The Rocks” in Sydney on Saturday. Saeed Khan/AFP

Her conservative state government was reluctant to impose the lockdown, but a growing number of health experts called for it, as Australia remains largely unvaccinated.

On Saturday, the case of a worker at the Granites gold mine in the Tanami Desert of the Northern Territory prompted the territory's authorities to order the isolation of more than 1,600 people in three states who had had contact with the worker.

The mine, owned by Newmont Corp, was put into lockdown.