SYDNEY (Reuters) - Australian health authorities say a new strain of the coronavirus in Sydney’s north most likely originated in the United States, but how it got from the airport to the community remains a puzzle. Australia had prided itself on a tough pandemic response including closing international borders and ordering lockdowns, resulting in a low mortality rate by global standards. But a week from Christmas Day a lengthy stretch without locally acquired infections ended when an airport driver and a couple - who had had no contact with each other - tested positive. Those cases have exploded into a cluster of at least 83, prompting a new lockdown in Sydney’s northern beachside suburbs and an investigation into whether and how the virus travelled from the airport 40km (25 miles) away. The authorities say the northeastern Sydney strain matches a positive test result of a woman in hotel quarantine who arrived from the United States, but how it crossed the city remains unknown. WHEN AND HOW WAS IT DISCOVERED? *After two weeks without a locally acquired coronavirus infection nationwide, a Sydney Airport transit driver tested positive on Dec. 16. *Also that day, a couple tested positive in the city’s Northern Beaches local government area. They had not been in contact with the airport driver. *By Dec. 21, there were 83 confirmed cases in the Northern Beaches, a community of 250,000 people sometimes jokingly called the “insular peninsula” due to a culture authorities hope will reduce wider transmission. *”The peninsula is a very cohesive community that tends to keep to itself ... and that is certainly ... assisting in making sure that the Avalon outbreak is staying exactly where it is,” Prime Minister Scott Morrison said on Monday, naming the suburb at the centre of the outbreak. *Most confirmed cases visited or had contact with someone who attended a Dec. 11 music event at Avalon Beach RSL Club, a pub run by the Returned and Services League, or a performance at Avalon Bowlo, a lawn bowls club, on Dec. 13. The authorities believe the virus spread through the community at these two events. HOW DID COVID-19 RETURN TO THE COMMUNITY? *Genome sequencing shows the Northern Beaches strain matches a strain found in a woman described as a returned traveller who arrived at Sydney Airport from Los Angeles Airport on Dec. 1, health officials say. *Authorities still do not know how the virus travelled from the woman to the Northern Beaches because she was in 14-day hotel quarantine at the time of the concerts. *The investigation will look into all contact with the woman in quarantine, including the possibility she touched the personal protective equipment (PPE) of a health worker. *The virus may have entered the community via “someone who came through the usual path but happened to be at that exact time ... exposed but not carrying a viral load that you could detect”, said Catherine Bennett, chair of epidemiology at Deakin University’s health school. WHAT IS AUSTRALIA DOING ABOUT IT? *The NSW government has put the Northern Beaches into lockdown until 11:59 p.m. on Wednesday. *The rest of Sydney’s five million people must limit visitors to 10. Entertainment venues have tighter limits on patron numbers and singing and dancing are banned. *The state has asked people to wear masks indoors but stopped short of making mask-wearing mandatory. *Australia’s seven other states and territories have reintroduced border closures, including road checkpoints, or mandatory quarantine for arrivals from NSW. HOW DOES THE OUTBREAK AFFECT CHRISTMAS? *The NSW government says it will make a decision on Dec. 23 about Christmas Day social distancing rules for the Northern Beaches and greater Sydney. *Most Australian states have waited for several weeks without community transmission before relaxing border controls, making it unlikely interstate travel will resume by Dec. 25.