SYDNEY • Melbourne began the first day of a six-week total lockdown yesterday, with the closure of most shops and businesses raising new fears of food shortages, as the authorities battle a second wave of coronavirus infections.

Shops were shut and streets were deserted in the city of about five million people.

Melbourne is Australia's second-largest city and the capital of Victoria state, which reported 471 new Covid-19 cases and eight deaths in the past 24 hours.

The country has now recorded about 20,000 Covid-19 cases and 255 fatalities, still far fewer than many other developed nations.

But the Victorian outbreak threatens to spill into other states.

New South Wales reported 12 new coronavirus cases on Wednesday, taking the national tally to 483.

There were no cases reported in other states and territories.

"We've flattened that curve once, we'll flatten that curve again," Health Minister Greg Hunt told reporters.

Victoria Premier Daniel Andrews urged Melbourne residents, who have already endured weeks of a less severe lockdown, to stay calm amid a surge in demand at supermarkets.

"There's no need for people to be trying to stockpile months and months of food," Mr Andrews told reporters in Melbourne.

Abattoirs are one of the few businesses allowed to stay open in Melbourne, though with a reduced workforce, under the stage four lockdown which took effect at midnight on Wednesday.

"We have tried to get that balance between reducing the amount of movement, therefore reducing the number of cases, but not compromising what you need being on the supermarket shelves," Mr Andrews added.

Residents are still allowed to go out during the day for exercise and food, or for work if their business is deemed essential.

While most residents appear to have heeded warnings and are keen for the lockdown to work, the measures have been opposed by some vocal critics.

Forecast national peak unemployment was revised upwards to about 10 per cent due to Victoria's re-imposition of restrictions, Prime Minister Scott Morrison said.

Counting those workers on Australia's wage subsidy scheme, Mr Morrison said effective unemployment would be closer to 14 per cent.

"These measures will have a very significant cost, and it will impact the recovery path," he told reporters in Canberra.

Mr Morrison warned that the latest lockdown would cost the Australian economy up to US$9 billion (S$12.3 billion).

He also said additional mental health support would be rolled out for Australians "working through the stress, and anxiety and strain" of lockdowns.

"Not being able to connect with friends and loved ones, concerns about employment, all of this takes a toll," Mr Morrison said.

Australia had previously forecast unemployment would hit a high of 9.25 per cent this year, as the economy endures its first recession in three decades.

After closing its international borders early, locking down cities and launching a campaign of mass virus testing, Australia had re-opened in June with daily cases in the single digit.

But hidden transmissions among staff at quarantine centres led to wider community transmissions in Victoria, which has recorded triple-digit new cases for weeks.

Victoria now has the bulk of Australia's infections, with more than 13,000 reported cases.

It reported a daily high of 725 new cases and 15 deaths on Wednesday.

REUTERS, BLOOMBERG