Prime Minister Christopher Luxon has dismissed calls from former National and ACT leader Don Brash for right-leaning voters not to give National their party vote, if they want to "keep the left out of our government".

ACT and New Zealand First have given their blessings to a newspaper advertisement - placed by Reality Check Radio, but written by Brash - encouraging voters to back the minor right-leaning parties.

Luxon said he "disagrees completely" with this approach.

"It's confusing messaging at the time of a tight election, and risks actually a Labour Party coming through and forming a coalition government," he said. "It's very simple.

"We want the National Party to have the biggest party vote, so that we can actually form a very strong and stable government in the years ahead. That's what it's about."

Brash encouraged people to split their vote and force an overhang, with National winning as many electorates as it could, but ACT and New Zealand First increasing their shares of the party vote.

In keeping with electoral law, both ACT and New Zealand First needed to sign off on the advert. On Friday, the parties confirmed they had done so.