A student from New Zealand's biggest school is in isolation after a person in their household tested positive for Covid-19.

The Auckland Regional Public Health Service has written to parents of Rangitoto College students saying the student is not a close contact, having only had minimal contact with the infectious person.

"The student is well and has had a negative test result. They have only been at school for a very short time since their household member became sick.

"Please be assured that the student was very unlikely to have been infectious with Covid-19 while at school. The whole family is in self-isolation and no one else from the household has been at school since their family member become contagious."

The risk to students and staff is "very low" and the school can operate as planned.

"Should we receive any further information about the level of risk, we will be back in touch."

The health service said as a "precaution" it was important to be vigilant for symptoms of Covid-19, including new or worsening cough, sore throat, runny nose, fever, temporary loss of smell and difficulty breathing. It urged parents, their children and other household members to call a doctor if any symptoms appeared, and to stay away from the school.

"If you know the identity of the student or family, please do not disclose their names or details to anyone else, and discourage your children from speculating or identifying them on social media. This can lead to online bullying and abuse."

Yesterday was the largest increase in a 24-hour period of active cases since August's outbreak with 25 new cases.

Two were new cases in the community after a 27-year-old port worker passed on the disease to his colleagues.

He is thought to have caught the disease on a visiting ship, the Sofrana Surville.

Last night all patrons and staff at The Malt Greenhithe last Friday night were told to get tested and self-isolate until they received a negative result, after one of the port workers spent around two and a half hours there.

Eighteen of the new cases were infected foreign fishermen staying in Christchurch's Sudima Hotel and five were unrelated imported cases.