NSW has another day without local COVID cases as authorities race to see if there has been another leak of the virus from hotel quarantine.

State health authorities are investigating transmission of the coronavirus from a positive couple to another returned traveller.

All stayed on the fourth floor of Sydney’s Radisson Blu hotel, prompting fears the virus could have spread while they were in quarantine.

A total of 21 former hotel guests and an unknown number of staff who worked at the hotel have been told to get tested for COVID and isolate pending further health advice.

Tweet from @NSWHealth

Genomic sequencing has shown all three cases have identical viral sequences of the Alpha strain (B.1.1.7).

The couple, who were asymptomatic, tested positive to COVID-19 on June 3, NSW Health said.

“At this stage we still don’t know if it happened on the plane, transporting from the plane to the accommodation, at the accommodation or whether it was just a coincidence,” Premier Gladys Berejiklian said on Wednesday.

The other returned traveller initially returned a negative test before subsequently testing positive two days later.

NSW Health said it remained unclear how and where the transmission occurred.

All three cases arrived into Sydney on the same flight from Doha on June 1 and stayed in adjacent rooms in the quarantine hotel.

NSW Health said the latest cases were not included in Wednesday’s figures because they are so far considered overseas acquired. One might become a local case, once the source of the infection is traced.

Elsewhere, NSW Health said it administered a record 17,223 COVID-19 vaccines in the 24 hours to 8pm on Tuesday, including 6048 at the vaccination centre at Sydney Olympic Park.

The total number of COVID-19 vaccines administered in NSW is more than 1.7 million.

-with AAP