SYDNEY - Australia will on Tuesday night (July 7) shut the border between its two largest states, New South Wales (NSW) and Victoria, following a worsening outbreak of Covid-19 in Melbourne that has forced hundreds of thousands of people across the city to go back into lockdown.

NSW and Victoria together account for almost 60 per cent of Australia’s population and are home to the cities of Sydney and Melbourne, which are the country’s commercial capitals. The flight between Sydney and Melbourne is typically one of the world’s busiest aviation routes.

But the flow of travellers will now come to an end for the first time since during the Spanish flu pandemic in 1919.

A sharp rise in the number of Covid-19 cases prompted the leaders of the two states on Monday to announce they will shut the border from 11.59pm on Tuesday.

Police officers, soldiers and drones will be used to patrol the border’s 55 land crossings. Typically, the road borders are marked by small signposts and many travellers would not notice that they had crossed state lines.

As of midnight on Tuesday, however, anyone who crosses the border faces an A$11,000 (S$10,680) fine and six months in jail, though permits will be granted for essential workers such as medical staff.

The Premier of NSW, Ms Gladys Berejiklian, said on Monday that no timeframe had been yet decided for the closure. She said the recent outbreak in Victoria largely involved community transmissions, whereas previous surges in Australia had mostly involved overseas travellers or their direct contacts.

“This is unprecedented in Australia,” she said. “We have not seen anything like this… We’re hoping this is a temporary arrangement. I want to see this happen for the shortest time as possible.”

Victoria, which has a population of 6.7 million, on Monday recorded 127 cases of Covid-19, eclipsing its previous high of 111 cases on March 28. For the past three weeks, the state’s case numbers have been rapidly increasing, even as the rest of the country has largely eliminated their outbreaks.

Victoria’s outbreaks have been linked to hotels for travellers in quarantine, where security guards are believed to have contracted the virus. It then spread through several extended families and neighbourhoods. The State Government has already put more than 30 hot spot suburbs into lockdown.

Australia’s Deputy Chief Medical Officer, Professor Michael Kidd, said on Monday the sudden rise in community transmissions in recent weeks showed “how quickly the pandemic can change”.

“The situation in Melbourne has come as a real jolt not only to the people in Melbourne but right across Australia who may have thought this was behind us – it is not,” he said.

Australia’s death toll from Covid-19 increased to 106 on Monday after two recent fatalities in Victoria.

Australia’s Prime Minister, Mr Scott Morrison, said on Monday the border closures would take an economic toll but was crucial to ensure Victoria suppressed its outbreak.

“It’s regrettable that this has been necessary,” he told 2B Radio. “We’re one country and that’s important and it’s important for our economy. But so is maintaining our strong health performance.”

The pandemic previously prompted other states to close their borders, but NSW and Victoria had remained open. These states had the highest number of cases, due to their larger populations and also because most international travellers arrive via Sydney or Melbourne.

But the sudden surge of cases in Victoria has taken the nation by surprise.

On Saturday, Victoria’s government imposed a lockdown on nine public housing towers which are home to about 3,000 people. Several clusters of cases have been linked to the towers.

But the sudden police-controlled restrictions raised concerns about the welfare of the residents, many of whom are socially disadvantaged and come from a range of cultural backgrounds.

A broad array of volunteers and charities has since come forward to offer food and other supplies.

Victoria’s Housing Minister, Mr Richard Wynne, said people across the state had offered support.

“Presently my phone is running off the hook with people saying, ‘How can we help?’” he said.