SYDNEY (REUTERS, BLOOMBERG) - Authorities in Australia stepped up contact tracing and called for people in the city of Melbourne to come forward for coronavirus testing on Thursday (Feb 11) as the number of infections in a cluster linked to a quarantine hotel rose to 11.

More than 22,500 test results were conducted in the past 24 hours in Melbourne, Australia's second most populous city, and authorities urged residents to get tested amid fears of community transmission from a worker at the Airport Holiday Inn in the city.

Three new cases were reported on Thursday, a manager at the hotel and the spouses of two members of staff, Victoria state authorities said.

All 11 cases in the cluster were confirmed to have the variant of the virus that emerged in Britain.

"Clearly, it is a very live outbreak," Victoria's Covid-19 response commander Jeroen Weimar told a televised news conference.

"We are at this stage reassured by the fact that all of these positives are emerging within the primary close contact field. We have a lot more work to do today, tonight and over the coming days."

British scientists have said the UK variant of the virus that is behind a surge in infections there may be not only more transmissible, but also more lethal - with a mortality risk around 30 per cent higher than other variants.

The outbreak in Melbourne, where the Australia Open tennis tournament is underway, has stirred fears of a fresh wave of infections in the state hardest hit by Covid-19.

Victoria had more than 20,000 cases last year and over 800 deaths, forcing authorities to implement a strict lockdown lasting more than 100 days, the most severe action taken by any Australian state.

The latest outbreak was likely sparked by a medical device known as a nebuliser being used by a Covid-positive guest at the hotel.

In total, there are now eight cases linked to the hotel: three members of a family who tested positive for the UK strain of the virus, two former guests and three hotel quarantine workers.

The remaining guests have been transferred to another hotel to serve out their quarantine and more than 100 hotel workers have been placed in isolation.

The nebuliser, which vaporises medication or liquid, also worked to spread the virus through mist "suspended in the air with very, very fine aerosolized particles," said Victoria state Chief Health Officer Brett Sutton.

This was how the virus was carried out of the hotel room into the corridor, where staff walking the halls were exposed, he said.

The outbreak is yet another example of the insidious ways that virus can spread, stymieing even countries with the strictest travel and containment regimes in the world.

It has raised concern that Australia's quarantine system, in which many overseas arrivals must stay in designated hotels in major cities for 14 days, must be reinforced or redesigned as more virulent strains of the virus spread worldwide.

Australia has been among the world's most successful countries in handling the novel coronavirus, largely because of decisive lockdowns and borders sealed to all but a trickle of travellers, with some 22,000 cases and 909 deaths.

But its quarantine hotels, where all international arrivals spend two weeks, have proved to be a weak link in its defences with the cluster in Melbourne the latest to emerge from one.

In response to the latest outbreak, South Australia state closed its border shut to Greater Melbourne on Wednesday, while Queensland introduced new border controls for anyone travelling from Victoria, the Sydney Morning Herald reported.

From 1am on Saturday (Feb 13), Victorians will again need to fill out border declaration passes when they enter Queensland, but the border will remain open.

The most populous state of New South Wales on Thursday reported zero new cases. Queensland and Western Australia are yet to report their numbers.

Australia was one of the first countries to call for an inquiry into the origins of Covid-19, which was first reported in the Chinese city of Wuhan, an action that sparked a diplomatic backlash from Beijing which has since slapped hefty duties or banned several Australian products from its shores.

The Sydney Morning Herald reported on Thursday that an Australian scientist, part of the World Health Organisation (WHO) team investigating the origins of Covid-19, told the paper he believed the virus began in China, despite WHO's official findings remaining inconclusive.

"I think the evidence for it starting elsewhere in the world is actually very limited. There is some evidence but it's not really very good," said Professor Dominic Dwyer, a microbiologist and infectious diseases expert with NSW Health.