A contact tracing expert says the Delta strain is shifting the goal posts on the country's tracing system, helping to put it under more pressure.
More tracers are being urgently recruited around the country as system struggles to keep up with the 14000 case contacts.
Otago University public health professor Philip Hill has been involved in reviews of the system in the past and says another investigation will be needed after this outbreak.
He told health correspondent Rowan Quinn that Covid-19 had previously been easier to contact trace than some viruses because it had a longer incubation period.
But Delta was changing that.