Public health officials have revealed 10 further Auckland bus trips taken by two people who later tested positive for Covid-19 - and are now racing to track down seven passengers who may have been exposed.

The two cases took a number of other buses while they were infectious, as well as a shared, two-and-a-half-hour journey on the 22N bus on Wednesday, August 12.

One case linked to the Auckland cluster took the 670 bus on August 10 along a route through Mt Roskill to Otahuhu and then later between Otahuhu and Avondale.

The other case - a Westfield St Lukes employee - also took buses on three days after the level 3 alert on August 14, 15 and 17, travelling on the same bus 22 route between Symonds St and St Lukes.

Seventeen passengers identified by their HOP cards as being on these buses at the same time as a case are considered close contacts.

They were now being asked to self-isolate and get tested, if Auckland Regional Public Health Service (ARPHS) could contact them through their HOP details.

Two people travelling on an unregistered HOP card on these routes on these days were close contacts, but authorities haven't yet found contact information for them.

People on these buses at these times, for longer than 15 minutes, were asked to self-isolate, call Healthline (0800 358 5453) for advice and get tested as soon as possible, ARPHS said in a statement.

"Anyone who was on one of these buses for shorter than 15 minutes at these times was considered a casual contact and should be aware there is a very small possibility that they have been exposed to the virus.

"As a precaution, they should watch for symptoms, calling Healthline if they become unwell."

On the day the two cases travelled together on the 22N bus, there was heavy traffic congestion ahead of the lockdown which came into force later that day.

"The situation was made worse with traffic queuing for the Covid testing centre by the St Lukes shopping centre," ARPHS said.

Six of the 11 close contacts on the bus at the same time have been alerted to their exposure and asked to be tested and go into self-isolation by the National Investigation and Tracing Centre.

Five people with registered HOP cards without up to date contact details have yet to be traced, along with two others on this bus who did not use a registered HOP card.

The 10 bus trips

Monday, August 10:

Bus number 670, between 9.13am and 9.39am, 91 Carr Rd, Mt Roskill (Stop 1365) to 28 Mason Ave, Otahuhu (Stop 7025)

Monday 10 August: Bus number 670, between 11.38 am and 12.31pm, from Otahuhu Town Centre (Stop 1356) to Rosebank Rd, Avondale (Stop 1456).

Wednesday, August 12: Bus number 75, between 9.44am and 10.04am, from 351 Remuera Rd (Stop 7415) to 83 Symonds St (Stop 7149)

Wednesday, August 12: Bus number 22N, between 10.30am and 1.04pm, from the Symonds Street Overbridge (Stop 7162) to New North Rd (Stop 8200)

Friday, August 14: Bus number 22N, between 08.33am and 8.47am, from 4 Symonds St (Stop 7146) to 63 Morningside Drive (Stop 8666)

Friday, August 14: Bus number 22R, between 6.06pm and 6.16pm, from 65 Morningside Drive (Stop 8861) to 25 Symonds St (Stop 7147)

Saturday, August 15: Bus number 22N, between 08.37am and 8.48am, from 4 Symonds St (Stop 7146) to 63 Morningside Drive (Stop 8666)

Saturday, August 15: Bus number 22N, between 5.11pm and 5.22pm, from 65 Morningside Drive (Stop 8861) to 25 Symonds St (Stop 7147)

Monday, August 17: Bus number 22N, between 10.04am and 10.16am, from 4 Symonds St (Stop 7146) to 63 Morningside Drive (Stop 8666)

Monday August 17: Bus number 22N, between 11.04am and 11.13am, from 65 Morningside Drive (Stop 8861) to 25 Symonds St (Stop 7147)