Labour's new deputy leader Kelvin Davis is a former principal, political battler, and the first Maori to take on the job.

Of Ngapuhi descent, he leads Labour's Maori caucus. His promotion to Labour's second most senior spot would give a "sense of satisfaction" not only to Maori MPs but to Maoridom, he said after his selection today.

In a strategic move, he was nominated for the role by Labour's finance spokesman Grant Robertson, a close friend of Ardern. Unlike Ardern, Davis is from the more conservative side of the party and represents a mostly rural electorate - Te Tai Tokerau.

He is a hard man with a sometimes aggressive style of politics, in contrast to Ardern's "relentless positivity" and warmth.

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He once confronted former Prime Minister John Key outside the debating chamber over his failure to protect Kiwis in offshore detention centres, calling him "gutless". Key was so rattled by the exchange that he furiously tore into Labour in the House, accusing them of "backing the rapists and murderers" - an outburst he later apologised for.

Davis prefers a background role, and admitted he would be "running the team" while Ardern was doing the "showing off" and taking the spotlight during the election campaign.

Raised in the Bay of Islands, he moved to Auckland in the mid-1980s to teach in Mangere. He later returned to the Far North and became principal of Karetu School - "a school with needs", he said.

Now living in Kaitaia, he is close friends with New Zealand First leader and Northland MP Winston Peters, a potential coalition partner, and Labour-turned-NZ First candidate Shane Jones.

Davis first ran for Parliament in 2008 against Mana leader Hone Harawira, and despite defeat he entered Parliament on the list. After failing to beat Harawira in a 2011 byelection and the 2011 general election, he left Parliament disillusioned and said he was quitting politics.

But he changed his mind and was given a political lifeline when Shane Jones resigned in 2014, allowing him to return to become an MP again as the next-ranked list candidate. He then sensationally beat Harawira at the fourth attempt in 2014 - an upset defeat that locked Internet-Mana out of Parliament.

Since his return, Davis has made an impression in the Corrections portfolio, where his accusations under Parliamentary privilege about prisoner mistreatment at the privately run Mt Eden Prison led to an inquiry and the eventual decision to drop Serco as its manager. The controversy was a key factor in the demotion of Davis' opposite at the time, former Corrections Minister Sam Lotu-Iiga.

His strong performance led to a promotion in Labour's caucus, but his elevation to the number two spot today still came as a surprise to the MP.

He hadn't even planned to be in Wellington for the caucus meeting which decided Labour's new leader. But in a text at 4.30am, he was told to catch the next flight from Kerikeri.

"It was the fastest shower I have ever had."

Asked about any ambitions to be the first Maori Prime Minister, Davis said possibly - after Ardern had held the role for "15 to 18 years".