"It's too early to say... we'll have a lot more information tomorrow," he said.
He played down one line of inquiry examining whether the virus was imported via freight, then picked up by a male member of the family, who worked in a cool room for imported goods.
"It's a possibility — it's unlikely but it's something we need to rule out," Bloomfield said, adding that another focus was whether the infection came from managed isolation facilities.
These facilities, mainly hotels and motels that have been sealed off from the general public, house thousands of new arrivals from overseas, with positive cases placed in stricter quarantine.
Bloomfield said that Covid-19 cases found in the community, along with some at-risk family members, were now being moved into quarantine as well for the first time, rather than being allowed to self-isolate at home.
"It will help us prevent any further inadvertent spread in the community," he said.
Despite New Zealand's previous success in containing the virus — with just 22 deaths in a population of five million — Bloomfield said health authorities had always anticipated such a return, even if it had taken some people by surprise.
"Yes we were becoming complacent and that's why our message over the last few weeks has been around avoiding that," he said.
"As you'll recall, last week I was talking about when — not if."
(Writers & Editors: NS/DM/KAF, Agence France-Presse)
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