Australia has enforced a three-day lockdown in Brisbane after a hotel quarantine worker tested positive for the more contagious variant of Covid-19 that emerged in Britain.

“We know that this UK strain is highly infectious. It is 70% more infectious, and we are going to go hard and we are going to go early to do everything we can to stop the spread of this virus,” state Premier Annastacia Palaszczuk told reporters.

The lockdown began on Friday and Australian chief health officer Paul Kelly said anyone who had been in the city since January 2 should also isolate.

It means the more than 2 million residents of Australia’s third-largest city will be barred from leaving their homes for anything but essential business.

Australia has found several cases of the variant but this was the first one to appear outside the quarantine system. Australia has also found a variant that emerged in South Africa in hotel quarantine.

It was not known if the hotel worker, a cleaner, was infected by someone who arrived from Britain but everyone would now need to show a negative test to travel to Australia, Prime Minister Scott Morrison announced.

“A negative test is not foolproof, but a positive test – they’re not coming,” Chief Medical Officer Paul Kelly told reporters in the capital, Canberra.

Morrison justified the Brisbane lockdown, given the danger that the new variant could quickly become the dominant strain of the virus.

“I know they will be some in Brisbane today asking ‘why is this necessary? There is only one case’. Well, this isn’t any ordinary case. This is a very special case and one that requires us to treat things quite differently,” Morrison told reporters.

Australia has had about 28,500 cases of the novel coronavirus and some 900 deaths, far fewer than many other countries, partly because of strict border controls from the outset of the pandemic.

Since March, the border has been shut to all non-citizens and permanent residents and the number of people allowed back each week has been capped at just over 6,000 people, with everyone going into mandatory hotel quarantine at their expense.

More than 35,000 Australians have registered their intention to return but Morrison said the number allowed back would be reduced by nearly half until mid-February, given the danger of the new variant.

“We can’t put it back in the box. We’ve got to act before we get the cases,” said the chief health officer of Queensland state, Jeannette Young.