Genome sequencing has revealed that a Sydney quarantine hotel cleaner who has COVID-19 contracted the virus at work.
A visibly relieved Health Minister Brad Hazzard broke the good news on Friday - a day after revealing the 26 COVID-free days in NSW had ended.
"It's a massive relief," he told reporters.
"The virus is not part of our locally transmitted viruses; it appears it has come from overseas," he said.
The revelation means COVID-19 could still have been eliminated in NSW but the question remains how the woman managed to become infected at work.
A NSW Health spokesperson on Friday afternoon said the infection likely came from a US source.
The source of infection may be international aircrew who were self-isolating in the hotel at the time, they said, however investigations are continuing.
Airline crews returning from overseas stay in quarantine hotels before turning around and going back overseas.
Mr Hazzard said all staff who work in quarantine hotels get weekly mandatory screenings and are tested for COVID-19 if they have symptoms.
NSW Health said the woman's case was the only local infection in the state in the past 27 days.
There were 11,128 tests reported in the 24-hours to 8pm on Thursday night with five cases in returned overseas travellers in hotel quarantine.
Deputy Chief Health Officer Jeremy McAnulty said testing of the woman's contacts was continuing.
"There have been no further positive results so far," he said.
Some staff at Sydney's Novotel Hotel who worked last Friday were being tested and placed in isolation.
Her colleagues who worked on November 28, 29 or 30 had already been tested and her five household contacts had tested negative.
Those who shared a train carriage with the woman are deemed close contacts and have also been asked to isolate for 14 days from their potential exposure.
A full list of affected train services and carriages is available on the NSW Health website.
The woman's infection sparked widespread alarm on Thursday, with fears Western Australia could withdraw its promise to open its border up to NSW and Victoria from next Tuesday.
WA Premier Mark McGowan says he will take the weekend to consider the situation.
However, Queensland authorities say its borders will remain open for now after travel restrictions were lifted on Tuesday.
"The government is keeping a close eye on what is happening in Sydney and our health experts are in regular contact with their NSW counterparts," Queensland Health Minister Yvette D'Ath said on Thursday.
Australian Associated Press