Five million people in Australia's second-biggest city began a new lockdown Thursday, returning to tough restrictions just weeks after they ended as Melbourne grapples with a resurgence of coronavirus cases.

Residents have been told to stay at home for six weeks after other measures to contain a spike in COVID-19 failed to prevent the virus spreading.

The state of Victoria has been effectively sealed off in an effort to preserve the rest of Australia's success in curbing the virus.

Australia enforces lockdown in Melbourne
Police stand guard outside a public housing estate locked down in Melbourne due to a spike in coronavirus cases. Photo: AFP / William WEST

However, a rush of travellers across the border into neighbouring New South Wales on Wednesday has raised concerns those efforts could be torpedoed.

"A few cases coming over the border from Victoria (can) tip that magic number into outbreaks that are going to be very hard to control," epidemiologist Mary-Louise McLaws told public broadcaster ABC.

New South Wales Premier Gladys Berejiklian said two cases of COVID-19 had been identified in the border town of Albury and she warned against travel to or from communities on the state frontier.

Travellers rush across the border
A rush of travellers across the border from Victoria into neighbouring New South Wales has raised concerns efforts to contain the virus spread could be torpedoed. Photo: AFP / William WEST

"We want to make sure that we're flushing out any potential seeding that occurred prior to that spike in cases becoming evident," she told reporters in Sydney.

Queensland state announced Thursday it would turn away all travellers from Victoria -- for the first time removing an option that had allowed them to spend 14 days in quarantine on arrival.

In Melbourne, there are concerns over the economic and mental health impacts of the second lockdown, which officials estimate will cost the economy Aus$6 billion ($4.2 billion).

Map of Melbourne, Australia
Map showing Melbourne, Australia where a new six-week lockdown is to be imposed starting today. Photo: AFP / STAFF

Restaurants and cafes are limited to serving takeaway food, while gyms, beauty salons and cinemas have been forced to close again.

Residents are restricted to their homes except for work, exercise, medical care or to buy essentials.

Despite the harsh measures, many said they support the restrictions.

"Lockdown was necessary, otherwise the cases wouldn't have reduced," resident Vani Kumar told AFP.

"Let's hope we all come out of it successfully," she said.

Authorities are waiting for more test results from high-rise public housing estates where roughly 3,000 people have been confined to their apartments since a cluster emerged Saturday, unable to leave for any reason in Australia's strictest lockdown.

There are fears the virus has spread across the towers, which officials have likened to "vertical cruise ships" and where many disadvantaged residents are reportedly in poor health.

Government workers are going door-to-door in Melbourne urging residents to get tested for the virus, with officials saying about 10,000 people have refused after being influenced by online disinformation.

Australia has recorded just under 9,000 cases of COVID-19 among a population of 25 million, with 106 deaths.

Outside of Victoria, the country has seen few new infections, with most cases involving returned international travellers in hotel quarantine.

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