By RNZ

There should be concern about the latest, small Covid-19 cluster say an epidemiologist and a Covid-19 modeller.

The country's second current cluster, now of six people and 31 close contacts, is connected to a man who was found to have the disease after he completed his isolation period and twice tested negative.

Two of the family of three who tested positive this week left managed isolation in Christchurch and returned home to Auckland on the same flight as a positive case.

Another family member then joined them on a trip to Taupō where they joined 18 others from Bay of Plenty, Wellington, Christchurch and Hamilton.

Work is still under way to trace the source of his infection, one possibility being that the man had an unusually long incubation period.

A Covid-19 modeller, Shaun Hendy, says the advice on isolation duration needs to be adjusted as more is learned about the disease and its possible incubation period.

"We don't know precisely when the infections took place, there's still the possibility that something went wrong, there was some mixing or some contact between people at the isolation facility," he said.

"There should be some concern about this latest case and I think particularly because it is distributed across the country, it's not confined to one part of the country."

But he says, "This is not the worst case".

Hendy says he doesn't expect many secondary cases of this cluster.

"We have caught this relatively early so the contacts of these cases, they'll be just entering the period about now when they'll be infectious."

Meanwhile, epidemiologist Professor Sir David Skegg told Morning Report the newest cluster has the potential to be very serious.

"This is just a reminder that elimination is a process, not a destination.

"This will keep happening and it's essential we can keep on top of the cases."

He says we cannot be complacent.

"We shouldn't think that we've beaten this thing, we haven't, it's raging around the world and it's going to keep cropping up in New Zealand from time to time."

Skegg doesn't believe managed isolation needs to be longer than 14 days.

But says there may need to be different, more strict procedures for people travelling from the UK, United States, Europe and India.