The Prime Minister says safety is key for the Pike River Mine re-entry.
Ardern spoke to media on Thursday afternoon, following the announcement from Andrew Little that the mine re-entry would be delayed.
She says the decision to delay the operation was a difficult, but the correct choice.
"It is no doubt deeply upsetting to have a delay like this because they've waited so long - but the families have always said they wanted a safe re-entry," she said.
The mine was due to be entered on Friday, but Little announced the delay on Thursday afternoon.
He said "unpredictable and unexplained" oxygen levels were found at the far end of the drift, and the shift in atmosphere means the air is no longer breathable.
Little explained the entry would not commence until here was certainty it was safe - a decision that has been supported by the widow of one of the victims of Pike River, Anna Osborne.
"Pike River played Russian roulette with our men - the recovery agency will not do the same with theirs," she said in a press conference on Thursday.