A Mount Albert Grammar student in Auckland who tested positive for Covid-19 was a close contact of the four existing South Auckland cases.
Auckland entered alert level 3 yesterday after four cases of the virus - all in one family - were detected in the community, breaking more than 100 days of no unknown sources of the novel coronavirus.
Director-General of Health Ashley Bloomfield yesterday said more than 100 close and casual contacts of the cases were being followed up, with four being probable and involving three adults and a teenager.
Dr Bloomfield confirmed to Morning Report that a student at Mount Albert Grammar tested positive yesterday and was a close contact of the family.
The school was the second largest last year, with about 3000 students.
Dr Bloomfield said the school notified staff, parents and students last night of the positive result after they were made aware.
"We got testing results all through last night on those [four probable cases] and other testing that was done," he said, adding that any further information on cases will be provided in the 1pm briefing update.
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The search for any more potential cases will be widened once again - as with all new cases - to find out where the person has been and who they've been in contact with over the past 14 days, he said.
"But at the moment, most of Auckland, many Aucklanders but very few should be at home ... and anyone who is a close and casual contact that we know about has already been asked to self-isolate."
While the original source of the virus remains a mystery, yesterday Dr Bloomfield noted that genome sequencing was underway to figure out the chain of transmission.
"I had an update last night that that was well in train ... you need to not only have done the genome sequence on the sample but then there's the scientific bit, where the specialists at ESR look at it and the exact sequence of different samples and see what the chain of transmission might be.
"The source of infection is of interest and it's important but the highest priority is for us to do this wide testing, particularly around close and casual contacts, to define the extent of the outbreak and manage it, only by doing that will be able to contain it.
"We will find the source, I have no doubt about that."
He said having Auckland at level 3 was helping with their efforts to trace and detect the virus.