The NSW health department is investigating how three members of a family acquired Covid-19 while in hotel quarantine in Sydney.
They were staying next door to a family with the virus at the Adina Apartment hotel.
The three cases within one family had been previously classified as overseas acquired but were reclassified on Sunday after NSW Health determined the cases shared the same viral sequence as the family of four who were in the adjoining room.
The families returned from different countries on different days, and NSW Health said the original family was believed to have been infectious between 8 April and 11 April, with investigations underway into how the transmission occurred.
All other guests on the 12th floor of the Adina Apartment hotel in the CBD returned negative results after being tested again, and staff who worked on the floor are self-isolating awaiting their results.
The families were both transferred out of the hotel into special health accommodation where they will remain until no longer infectious.
Transmission between people staying in hotel quarantine has been an ongoing struggle for state governments managing the programs. The Victorian government put the state into a five-day snap lockdown in February after one guest passed on Covid-19 to other guests and staff at the Holiday Inn.
Victoria only began re-accepting international arrivals from 8 April after another review and overhaul of hotel quarantine, and now has close to 100% of staff working in the program vaccinated against Covid-19.
In Queensland, there have been two outbreaks linked to the Grand Chancellor Hotel in Brisbane where returned travellers are quarantining.
In January, the more contagious UK variant spread from returned travellers to a hotel cleaner, sparking a three-day lockdown.
In March, a Grand Chancellor Hotel guest passed the virus to a doctor at the Princess Alexandra Hospital, forcing the hospital into lockdown.
NSW reported six new cases of Covid-19 overnight, all overseas acquired in hotel quarantine. There were 8,088 tests reported, bringing the total number of vaccines administered by NSW now to 173,852.