There has been too much comment about how Jacinda Ardern looks, Labour MP Annette King says.

King, who has been a mentor to Ardern and was replaced as deputy leader by her, said those who dismissed Ardern as a lightweight were in for a shock now she was Labour leader.

"There has been too much comment about how she looks. And that has overshadowed what is a really sharp intellect and huge compassion and commitment to politics and policies.

"I think it is interesting - people talk about her smile, her teeth or her hair or whatever. And haven't looked at the qualities of her. You don't see that happening when they analyse other politicians."

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King said Ardern had steel and a keen sense of humour.

"That can actually get politicians out of a lot of trouble. And actually it's something that hasn't been on display for a long time in a politician. Certainly compared to Bill English - she is the genuine thing, she won't try and pretend that she likes spaghetti on top of her pizzas."

Labour veteran Trevor Mallard said Ardern's media performance today showed she had the policy depth of Helen Clark and the wit of former Prime Minister David Lange.

"I haven't seen that level of competence in a potential Prime Minister for a very long time," Mallard said.

"She is fiercely intelligent, very much like Helen Clark in that way, but I think she has also got the advantage of youth and vibe, and an ability to connect - probably better than Helen did before Helen was PM.

"Something I have noticed when I've been door-knocking is a lot of the older people who have watched her on breakfast TV earlier in her career really love her. And I think as she gets that exposure she will do really well."

The Labour MPs Ardern is particularly close with include Grant Robertson, Chris Hipkins and Annette King.

Robertson said Ardern was fiercely intelligent and a great communicator, with a sharp sense of humour and "one of the most caring people I have ever met".

That warmth shouldn't be equated with weakness, Robertson said.

"She is a real fighter. People have seen her and appreciated her as being warm and funny, but she has huge spirit."

Ardern has had a steady rise through the ranks of the Labour Party since entering Parliament in 2008. Born in Hamilton in 1980, she attended primary and secondary school in Morrinsville, also spending a few years living in Murupara, where her father was a policeman.

She joined Labour aged 17. She grew up in a Mormon household but left the faith in her 20s, mostly because of its anti-homosexual stance.

After completing a Bachelor of Communication Studies at Waikato University, Ardern worked for Phil Goff and Helen Clark.

She spent three years in London as a policy adviser in the UK Cabinet Office, and in 2007 was elected president of the International Union of Socialist Youth.

Ardern, who has cited Clark as a political hero, stood for Labour in Waikato in 2008 and entered Parliament as a list MP. She was narrowly defeated by now Education Minister Nikki Kaye in Auckland Central in 2011 and 2014.

After moving into Mt Albert with her partner, broadcaster Clarke Gayford, Ardern stood in and won the Mt Albert byelection in February, prompted by the resignation of David Shearer.

Her strong showing in that contest saw speculation she would be elevated to the deputy leader position. After Annette King stood down from the position Ardern was elected unopposed by Labour's caucus. At the time, Andrew Little said it was partly because of her ability to reach a group of people the party needed to connect with better - young and urban.

Ardern has never stood for the Labour leadership in the three contests since 2011 but she was on Grant Robertson's unsuccessful ticket as deputy following the 2014 general election.

She is spokeswoman for justice, children, arts culture and heritage, and small business, and associate spokeswoman for Auckland issues.

Ardern has steadily risen in preferred Prime Minister polls - out-performing Little - and is the highest-ranked Opposition MP by chief executives surveyed in the Herald's Mood of the Boardroom.

Jacinda Ardern and other young leaders

Jacinda Ardern was confirmed as Labour's new leader just under a week after she blew out the candles on her 37th birthday cake.

In her first press conference Ardern joked she was "youth-adjacent". So, how does her relatively young age compare to when other leaders took over?

Helen Clark was 43 years old when she became Labour leader in 1993 and 49 when she became Prime Minister.

Prime Minister Bill English wasn't much older than Ardern when he became National Party leader in 2001 - he was 39 years old.

Canadian Prime Minister Justin Trudeau was 41 when he became leader of the Liberal Party, and 43 when he became Prime Minister.

Emmanuel Macron became the youngest President in French history when he was elected at the age of 39.

William Hague was elected as leader of the British Conservative Party aged just 36. Tony Blair took over as leader of British Labour aged 41, and became Prime Minister aged 43.