Jacinda Ardern squandered the traditional challenger's advantage of her first leaders' TV election debate with Bill English, political commentator Vernon Tava has suggested.

Tava told ZB's Early Edition host, Rachel Smalley, that the challenger to the prime minister had usually won the first of the televised debates in the last few general elections, but he suggested last night's first-round sparring ended in a virtual draw.

"It was a very close result and I just think they were very evenly matched, probably a lot more evenly matched than a lot of people may have expected," Tava said.

"Traditionally, though, the challengers do normally 'win' ... the first debate."

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Past Labour Party leaders Phil Goff and David Cunliffe were considered to have won the first televised election debates against then-Prime Minister John Key.

"This is usually the way things go. I think it will be the following debates that really count as far as who comes ahead."

Tava said the 1 News-Colmar Brunton poll result putting Labour ahead of National - "the best poll result for Labour in a decade, over a decade" - "I think it may have rattled English a little, although neither party is totally convinced that this is where the result will settle".

English smiled more than usual and warmed up as the debate progressed, while Ardern looked "more serious and prime-ministerial than usual".

"Bill, he focused very much on the vagueness and lack of clarity in the tax detail that's been offered by Labour and, you know, he came up with some zingers. 'People can't go shopping with your values', was probably the line of the night overall."

English displayed "more mastery of the detail. He didn't fall back on the statements of values and the higher level ... principles".

And he shone in "articulating the vision for social investment which I think has become his real strong suit throughout the campaign, talking about the real specifics of how the Government is going to invest in a targeted fashion for better outcomes".

Ardern had talked a lot about "my generation" and "your generation", Tava noted. She was clearly framing herself as a representative of generational change.

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Her talking about values rather than detail would have appealed particularly to younger voters.

She insisted she was being transparent and clear, "but sometimes that was about her refusal to reveal any more detail on tax policy and it did start to grate a bit as time went on and she was left unable to answer some questions".

"The most striking, apart from tax, was the immigration question. That really ended up in a non-answer."