NSW coronavirus contact tracers have used a “variety of mechanisms” to identify passengers who travelled with an infected taxi driver, but have been left stumped by nine of his trips.

At the weekend, health authorities revealed the Silver Service driver worked eight days in several areas around Sydney’s west and south-west earlier in September.

NSW chief health officer Kerry Chant said authorities had already identified “a large number” of passengers who might have been exposed to COVID-19 in the cab from September 7 to 10, and 14 to 18.

“We have been urgently attempting to contact anyone who undertook trips with that taxi driver,” Dr Chant said.

“We have managed to use a variety of different mechanisms to identify a large number of the people who shared a ride with that taxi.”

NSW Health identified four new coronavirus infections in the 24 hours to 8pm on Sunday, three of whom were returned travellers in hotel quarantine.

The other case was linked to the Concord Hospital cluster and was already self-isolating.

Meanwhile, NSW Health has used a combination of credit card transactions, trip data and booking information to contact many of the passengers who travelled with the infected taxi driver.

However, people from nine trips have still not been traced.

“The ones that we are less conscious of are those [passengers] who might have hailed the cab driver and it’s not registered in some way,” Dr Chant said.

The trips are:

• September 8: Liverpool TAFE to Hoxton Park Road, Cartwright (13:03-13:14)
• September 8: Liverpool TAFE to Glenwari Street, Sadlier (14:01-14:11)
• September 9: Liverpool TAFE to Moorebank Shopping Centre (08:08-08:15)
• September 10: Liverpool to Graham Avenue, Casula (14:38-14:44)
• September 10: Haddenham Street, Chipping Norton to George Street, Burwood (19:15-19:48)
• September 14: Riverside Road, Chipping Norton to The Mill Hotel, Milperra (15:50-16:04)
• September 15: Riverside Road, Chipping Norton to Birnie Avenue, Lidcombe (08:31-09:11)
• September 15: Milperra to Riverside Road, Chipping Norton (10:14-10:25)
• September 15: Birnie Avenue, Lidcombe to Riverside Road, Chipping Norton (15:22-16:07)

Dr Chant said all passengers who had travelled with the infected driver were considered close contacts and had to self-isolate for 14 days and get tested for the virus.

She said authorities were working with the taxi and ride-sharing industry about introducing a “check-in” system, similar to restaurants and other venues, to make passengers easier to trace.

“Sit in the back, sit diagonally opposite the driver and also wear a mask when you’re in a cab,” Dr Chant said.

-ABC