Two people later diagnosed with Covid-19 travelled on the same Auckland bus.
The Regional Public Health Service has found the people were not travelling together, but one is believed to have picked up the virus from the other.
Auckland Transport has confirmed there were 16 other passengers who took the same route, plus the driver.
Eleven are considered close contacts.
It has been able to trace nine of them using details from their HOP cards.
Anyone who took the 22N bus on 12 August between 10.30am and 1.04pm is advised to self-isolate and get tested.
The Public Health Service is urging all public transport users in Auckland to register their travel cards.