National is accusing Chris Hipkins of "playing politics" following his call for a ceasefire in Gaza and Israel.
Earlier on Sunday Hipkins, speaking as party leader not caretaker Prime Minister, broke protocol by saying it was untenable to sit by and watch the "horrific scenes" without calling for it to stop.
He added he was "particularly concerned" that the actions of Israel's defence forces are "disproportionate and indiscriminate".
And he blamed the lengthy coalition negotiations for not speaking out earlier.
On Sunday hundreds of protesters descended on Parliament, which is still without a Government, to call for a ceasefire in Gaza. Inside, that's exactly what Chris Hipkins did.
"I, and the Labour Party, cannot stand by any longer in the face of the horrific scenes we are witnessing without calling for a ceasefire," Hipkins said.
"We are urgently calling for a ceasefire. Israel and Hamas need to immediately ensure the conditions for a ceasefire are met and to commit to a lasting peace in the region."
As caretaker Prime Minister Hipkins is only supposed to make statements agreed to by the incoming Government.
"It has become untenable for me to do that and not speak about this situation," he explained.
So instead he said he was speaking as Labour leader - and not on behalf of the incoming Government.
He said this was because the caretaker Government had sought agreement from the Opposition for a ceasefire - however that's "not something they'd agreed to".
"I would prefer New Zealand to be more assertive in the position we have been taking," he said.
Hipkins unwilling to stay silent any longer - and taking a jab at National for the time it's taking to form a new Government.
"We're now five weeks out from a general election and the shape of the Government is no clearer and there is no end in sight to those negotiations," he said.
But Hipkins' statement has riled up National, with a spokesperson accusing him of "playing politics" with such a serious issue.
"On Friday afternoon the caretaker Government approached National about calling for a ceasefire," they said in a statement.
"In response, National asked to see MFAT advice on the matter - we provided feedback on that advice and indicated we were open to a discussion with Labour on it.
"National was then informed of the Labour leader's statement four minutes before the press conference commenced."
The spokesperson added that National supports the goal of a ceasefire, however the conditions haven't yet been right for one.
"If reports of a possible temporary ceasefire being close are correct, with hostage exchanges from both sides and humanitarian aid into Gaza, this is what New Zealand has consistently called for," they said.
"It is hoped that any temporary ceasefire could last longer than five days and lead to peace talks."