Kelvin Davis says he had no idea he was about to become Labour's deputy leader until immediately before the party's crisis meeting on Tuesday morning.
Mr Davis and Jacinda Ardern were bloodlessly elected the party's new leaders after Andrew Little stepped aside, following a string of poor poll results.
"I didn't know I was part of a ticket with Jacinda [until] just before caucus," Mr Davis told The AM Show on Friday.
There were text messages "toing and froing" before sunrise that morning which gave him a hint he may have been in line for a promotion - confirmed when he was approached on his arrival in Wellington.
"Sometimes there's a moment where you have to say either yes or no and it could go one way or the other, and I just said, 'Yeah, hell, why not?'"
The reaction from pundits and on social media has been largely positive, with many citing Ms Ardern's youth and star power - especially when compared to her counterpart in the National Party, Bill English.
"That warm glow, everything sugar and spice and everything nice just emanates from Jacinda, and we look over there and all we get is a dull ache," said Mr Davis.
Māori Party on-side?
The change in leadership has opened up the possibility the Māori Party could switch sides too. Co-leader Marama Fox phoned into RadioLIVE talkback late Thursday night, saying the leadership switch is a "phenomenal move - the best tactical move they've made".
"Labour have turned their backs on Maori, but finally with Kelvin as the deputy... they have recognised the value of the Māori vote to the labour movement. It has been a long time coming," she said.
The last three elections have seen the Māori Party pragmatically side with National.
"Our party membership do not want us to be sitting in opposition," said Ms Fox.
If Labour, the Greens and NZ First - or some combination thereof - are able to put together a viable Government, the Māori Party would keep that pragmatism going.
"We could command a Government that is Jacinda Ardern, Metiria Turei and Marama Fox, with all of our co-leaders and deputy leaders in tow."
The Greens got a recent boost in the polls following co-leader Metiria Turei's welfare fraud admission, but new revelations could see those gains lost.
Labour dropped to a record-low 24 percent in the latest Newshub-Reid Research poll, but will be hoping Ms Ardern's appointment will claw back voters from a National Party gunning for its fourth consecutive term in power.