Students from two Auckland schools have ridden the same school bus as Marist College students with coronavirus while they were infectious.
A "handful of students" from Lynfield College and Mount Albert Grammar were now considered casual contacts, parents were told yesterday.
Thirteen students and staff at Auckland's Marist College have now tested positive for the virus.
Seven teachers, including principal Raechelle Taulu, and four students have all tested positive, Marist Girls' College board of trustees chairman Stephen Dallow said. Details of the other two cases had not been released.
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Mount Albert Grammar headmaster Patrick Drumm told parents students aboard nine Kiwi Coach bus rides last week are now at risk of contracting coronavirus.
"While this will be concerning for you, Auckland Regional Public Health Service considers most of the students on the buses to be at low risk of catching Covid-19," Drumm said.
"Between four and six students who sat near the infected student on each bus are at slightly higher risk, but it is impossible to identify who these students are."
The bus rides were:
• The Mangere Bridge bus with Lynfield College students on Monday, March 16, in the afternoon, and Wednesday, March 11, morning and afternoon.
• The Manurewa bus shared with Mount Albert Grammar in the morning and the afternoon from Wednesday, March 18, to Friday March 20.
Director-General of Health Ashley Bloomfield today said one of the suspected clusters of community transmission was at Marist College.
As alert level 4 began at 11.59pm on Wednesday, these students are now in lockdown with the rest of the country.
"We ask that they take this seriously and remain inside, with no visitors," Drumm said.
"They should not be socialising with others - even at a distance, and they should not go out for essential supplies to the supermarket or pharmacy for 14 days."
There was a small possibility that students aboard these buses will develop symptoms, such as a fever, cough or difficulty breathing.
"In this case, please ring Healthline on 0800 358 5453 and tell them that your child is a contact of a Covid case," Drumm said.
"They will advise you what to do."
Mount Albert Grammar students were asked to respect the privacy of the Marist students and not to speculate about or confirm who they were through text messages or on social media.
There are now 283 confirmed or probable cases of coronavirus in New Zealand, with 73 new cases announced yesterday.
Twenty-seven people infected with the virus have recovered, Bloomfield said.
Bloomfield said the number of confirmed cases could peak in the thousands, but he hoped the turnaround point may be in 10 days - if people complied with the lockdown rules.
That was based on a number of models, which will be released once ministers had seen that information.
"We will expect the number of cases to increase for at least the next 10 days. If we all do our bit, and we break the chain of transmission, slow the spread of Covid-19 and we will be able to get on top of this."