Jacinda Ardern lacks substance compared to Bill English - Paula Bennett

Paula Bennett says new Labour leader Jacinda Ardern has "stolen" her "relentless positivity".

Ms Ardern's first move as leader was to ditch the party's old campaign slogan of 'A fresh approach', replacing it with 'Let's do this'.

"It's not just about the next seven weeks - it's about what we can do if we are in a position to make change," she told media on Friday afternoon.

"I'm excited by this campaign and by what's possible - so let's do this."

Previous leader Andrew Little was branded by right-wing bloggers as 'Angry Andy' - a label they might struggle to pin on Ms Ardern.

Deputy Prime Minister Ms Bennett, speaking to The Nation on Saturday morning, said "relentless positivity" is core to her own outlook.

"I feel that's what I am. There we go. I'm putting that on the table. I feel like she's stolen my relentless positivity, so I'm going to go for eternal optimism."

That positivity will likely result in a "wee bump" in the polls, says Ms Bennett.

But she believes Ms Ardern lacks two key things Prime Minister Bill English has - substance and "the kind of brain to pull this country together".

"I just think he's got a bit more is all," Ms Bennett told host Lisa Owen.

"I think she does, but I think that he equally has a proven track record, that he's got a strength in other areas, that he is a good communicator, that he's passionate about this country, and there's some pretty hard calls you have to make as being the Prime Minister, and we've had some pretty shocking things that have happened in this country just in the last few years, and so we want a leader and a Prime Minister that's actually up to that job.

"You're not going to have a lot of time to get to know her under that kind of pressure in just a few short weeks."

Despite Ms Ardern's perceived shortcomings, Ms Bennett says National will be taking her seriously.

"I would take her as seriously as I have any of the previous five before her that I've seen in the last very short years. So I think that she's got strengths in some areas and weaknesses in others that we all do, but we definitely take anyone that's leading an Opposition party seriously."