"I'm not going to get into the details of that - this is a puzzle and the reason it's a puzzle is that we don't have a known or identifiable link to people who are in a more high-risk category," he told the radio station.

In a separate interview with The AM Show, Hipkins said testing at the case's workplace was undertaken on Wednesday.

He said contact tracing will continue until the source of the outbreak is found.

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"More testing will be taking place today [Wednesday] and we'll also be doing a surge of testing across the Auckland region - we expect to see very high testing levels over the next 72 hours."

Ardern told reporters on Tuesday night that putting Auckland into lockdown was a precautionary approach but one that needed to be taken. 

Hipkins said the cases were a puzzle - one that needed solving.

"The contact tracing so far has not revealed any contact with someone who has been working at the border or who has recently travelled, or who has been in or worked in managed isolation or quarantine," he told The AM Show.

Earlier, National leader Judith Collins acknowleged the move to alert level 3 was a necessary one.

"I think it has to happen if there's been a community outbreak - certainly in a family anyway," she said.

"This is actually quite a serious situation. I'm very hopeful that they can find out how this has happened but it's [COVID-19] clearly come in somewhere."