Officials in Australia’s second-largest city blamed the nation’s slow jab rollout for the decision to increase restrictions.
Just 20 per cent of Australian adults had been fully vaccinated by Wednesday.
It joins Sydney and Brisbane in locking down in a bid to contain the highly contagious variant.
Melbourne and surrounding Victoria state will lock down for seven days after eight new infections were detected in the city, Victoria premier Daniel Andrews said.
Mr Andrews gave less than four hours notice about the restrictions, which came into force at 8pm local time.
“To be really frank, we don’t have enough people that have been vaccinated and, therefore, this is the only option available to us,” he said.
“The time will come when we have many more options. But that isn’t now.”
He also accused neighbouring New South Wales state of taking too long to lock down Sydney. It follows a limousine driver testing positive for the delta variant after transporting a US aircrew from Sydney Airport on June 16.
On Thursday New South Wales reported its worse day since the Sydney lockdown began on June 26, recording a record 262 new local infections and five deaths.
New South Wales premier Gladys Berejiklian said four of the dead had not been vaccinated. One had had a single dose of the two-shot AstraZeneca vaccine in late May.
Australian authorities have urged people in Sydney not to wait the optimal 12 weeks before taking their second AstraZeneca dose.
Ms Berejiklian said: “No one who has died has had both doses of vaccine. I cannot stress enough how it’s so important for everybody of all ages to come forward and get the vaccine.”
AstraZeneca and Pfizer are the only vaccines available in Australia.
There have been 21 Covid-19 deaths in Sydney since the latest outbreak began and 78 deaths overall in New South Wales since the pandemic began.
When Victoria ended its fifth lockdown last week, Mr Andrews said he believed the state was the only jurisdiction in the world that had beaten a Delta outbreak twice.
Melbourne was at the centre of the Australian pandemic last year, when new infections peaked at 725 in a day in August. Of Australia’s 925 deaths linked to the virus since the beginning of the pandemic, 820 have occurred in Victoria.
On Thursday authorities in Brisbane said they were confident that an eight day lockdown the city and surrounding areas in Queensland state will end as planned on Sunday.
Queensland Chief Health Officer Jeannette Young said efforts to suppress the spread had surpassed expectations.