Dr Ashley Bloomfield has warned more cases of COVID-19 may emerge with links to bus trips in Auckland but has praised alert level 3 for reducing potential exposure.
On Monday the Director-General of Health announced eight new cases from community transmission - one of which had contact with another confirmed case on a public bus.
There are now three confirmed cases of coronavirus who used public transport before testing positive.
During the 1pm COVID-19 update, Dr Bloomfield was asked about the likelihood of more cases extended from those bus trips.
"The exposure probably happened around 10 days ago now, so there may still be some cases that come through," he said.
"The idea is to try and proactively contact people who had been on those buses at the same time as any one of the now three cases that have gone through from a bus trip. We may well see further cases."
The Ministry of Health has been using data from Auckland Transport's Hop cards to track the other bus trips the cases had taken, which were made publicly available on Sunday.
But Dr Bloomfield said that having the city in lockdown has limited the potential exposure of the virus.
"The important thing is this is happening at alert level 3 in Auckland so there is much, much lower rates of movement and people aren't going out and about as much," he said.
"For example, the new case which is from a contact on the bus, that person had been working at home ever since they took that bus trip. Hence the value of that alert level 3 is that people are not out and about doing what they would have."
One of the major factors considered by the Government on whether to end or extend Auckland's lockdown was about the perimeter of the cluster.
When asked how confident he is that it has been obtained, Dr Bloomfield said it was encouraging the "vast majority" of the cases could be linked to close contacts.
"But certainly these other cases including the one on the bus, and there are now two linked to that, do give us pause for thought as indeed does the case that presented at North Shore Hospital," he said.
"We have, right since the start, we had cases where we initially couldn't epidemiologically link them, in fact, one of our first cases who was the GP, we have a genome sequencing link to the outbreak but we still have not epidemiologically been able to link that.
"It is important that we've got that perimeter identified so there will be ongoing high rates of testing this week in particular in the Pacific community in south and west Auckland."
