Jacinda Ardern doesn't change the game - Bill English

Bill English thought the election would be tight when Andrew Little led the Labour Party, and the rise of Jacinda Ardern hasn't changed that view.

"It hasn't changed our confidence at all," the Prime Minister told The AM Show on Monday morning. "We've yet to see whether the change in leadership changes Labour's vote much, whose expense it comes at. Either way, we've always said it's going to be a tough election to win."

He says National hasn't done any polling of its own since Ms Ardern took the leadership six days ago, but expects Labour to get a boost.

"I'd imagine Labour's numbers will go up a bit - it's seven weeks until the election though."

Ms Ardern drew a strong crowd on Sunday, launching Labour's Auckland transport policy - even some Green MPs turned up. Mr English will be hoping any voters drawn to Labour will be coming from the Greens, and not National.

"Labour and the Greens and NZ First, they'll be mixing and matching and they've got their own little circus going on. We have to get our support up."

On the most recent polling, National would struggle to form a Government with its current support partners ACT, United Future and the Māori Party.

Labour and the Greens could, but only with the support of NZ First. To do it without NZ First, Labour would have to lift its support to levels not seen since Helen Clark was in charge, all the while not taking any from the Greens.

"I'm sure Ms Ardern is a competent politician," says Mr English. "We'll all be tested over the next seven weeks - that's what elections are for."