The prime minister first learned minister Meka Whaitiri plans to resign from Labour to join Te Pāti Māori when he arrived in the United Kingdom.
Whaitiri, who has been MP for Ikaroa-Rāwhiti since a by-election in 2013, made the announcement at a marae in Hastings on Wednesday morning.
Speaking in London just before Whaitiri's official announcement, Chris Hipkins said he had tried to contact Whaitiri but was yet to speak to her.
"I first became aware of the ... speculation when I arrived here in the UK, I've left a message for Meka. I would expect if there was anything significant happening in that area that she would give me a call," Hipkins said.
"I certainly know it's not been raised with me. I've not had any conversations with Meka about it. She's not really raised any concerns with me, either. And obviously, as a member of our team, you know, I interact with all of our MPs on a regular basis."
Hipkins said he last spoke to Whaitiri at Labour's last caucus meeting before the Easter recess.
MPs often consider their future plans leading up to elections, Hipkins said.
"Typically, they speak to their party leaders before making those announcements. I haven't heard from her yet," he said.
He was waiting to see what she has to say, "if anything", before commenting further.
Whaitiri was sacked from her ministerial portfolios in 2018, following allegations she bullied a staffer.
She was reinstated a minister outside Cabinet in 2020, and currently serves as minister of customs, veterans, and food safety.
The MP, ranked number 20 in the Labour caucus, also has ministerial responsibility for the Hawke's Bay cyclone response, following the sacking of Stuart Nash.
RNZ has contacted Whaitiri for comment.
Before the move was confirmed, Te Pāti Māori president John Tamihere did not deny the rumour, saying "all will be revealed" Wednesday morning.
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Green Party co-leader James Shaw, also a minister outside of Cabinet, said Whaitiri would have to resign her portfolios.
"Obviously if it is true, if the reports are correct, then it is quite significant."
Former Cabinet minister Tau Henare represented three different parties during his parliamentary career. He quit New Zealand First in the late 1990s to form Mauri Pacific, then returned in the 2000s as a National MP.
He told Morning Report he heard about the "bombshell" on Tuesday night.
"I've heard rumours, but rumours in politics are an everyday thing."
Henare was not told any reasons for Whaitiri's reported move, but suspected it was her lack of career progress. Whaitiri was ranked 19th on the Labour list in 2014, and 28th in 2020, but has not been a Cabinet minister since 2018.
"But at the end of the day, I think that her treatment - and I don't say it was bad treatment - but her treatment in the reshuffle and just not moving on up, was a sort of catalyst… I think it's a pretty shrewd move, to be honest."
He agreed with Shaw's assessment she could no longer serve as a minister.
"A lot of us who've jumped ship and whatnot were backbenchers, middle-benchers, so it's not as much a bombshell. This one, she's a serving minister. She gives up her ministerial post… to join another party."
But since this was close to the general election, no byelection would be triggered.
Henare said it was a "shrewd" move that would boost Te Pāti Māori's chances of making it back into Parliament, with polls putting them below the 5 percent threshold needed without an electorate seat.
It would also help Labour, ironically, given that Te Pāti Māori - if kingmaker - were unlikely to side with National "even in their wildest dreams", Henare said.