SYDNEY (AFP) - More than six million people were ordered into a snap, five-day lockdown Friday (Feb 12) to contain a new coronavirus outbreak in Australia's second most populous state, forcing the Australian Open tennis tournament in Melbourne to shut fans out.

The lockdown across Victoria state from midnight (1300 GMT) was ordered out of fear a small outbreak of the "hyper-infectious" UK strain of the virus would undo Australia's so-far successful battle to contain the pandemic, Victoria state Premier Daniel Andrews said.

The Victoria state government also announced a suspension of international passenger arrivals into Melbourne from 11.59pm on Saturday (Feb 13) until further notice.

As a result, Singapore Airlines announced on Friday night that three passenger flights from Singapore to Melbourne will be converted to cargo-only flights: SQ217 departing Singapore on Sunday (Feb 14), and SQ237 departing Singapore on Monday (Feb 15) and Tuesday (Feb 16).

SIA flights from Melbourne to Singapore will continue to operate as scheduled.

The lockdown dealt another blow to the year's first Grand Slam, which started three weeks late to allow international players to quarantine and had already welcomed tens of thousands of socially distanced fans in the biggest crowds seen in tennis since the pandemic.

Under the lockdown restrictions, people will only be allowed to leave their homes for a limited number of "essential" activities like shopping and exercise, and public gatherings were prohibited. Face masks are mandatory when outside the home.

It was a devastating setback for residents and businesses who underwent a similar lockdown in and around Melbourne for more than 100 days late last year in a successful effort to contain an earlier outbreak that infected thousands and killed about 800.

Small business owners were all the more distraught as the measures come over the Valentine's Day weekend, when they were counting on big profits to help recover from the earlier lockdown.

"These restrictions are all about making sure that we respond appropriately to the fastest-moving, most infectious strain of coronavirus that we have seen," Mr Andrews told a hastily organised press conference Friday after the number of cases linked to a quarantine hotel at Melbourne airport rose overnight.

"I am confident that this short, sharp circuit breaker will be effective. We will be able to smother this. We will be able to prevent it getting away from us."

Australian Open director Craig Tiley gave assurances that the tournament would continue, with players deemed essential workers exempt from lockdown, competing in a "bubble form".

"The players have all been very good about it. They understand. They have been through a rigorous programme already," he added.

The problems at the Australian Open, the biggest international sports event so far this year, underline the difficulties of holding the much more complex Tokyo Olympics, which are due to start in July after a year's delay.

Play at six warm-up tournaments in Melbourne was also suspended for a day last week when a worker tested positive at one of the official hotels.

Two other Australian state capitals, Brisbane in the east and Perth in the west, recently underwent similar snap lockdowns in response to cases of the UK strain leaking from hotel quarantine.

In both instances, the outbreak was quickly contained.

Australia has been among the world's most successful countries in containing the coronavirus, with some 900 deaths for a population of 25 million.

With additional reporting by The Straits Times