Opposition leader Simon Bridges says he was forced to publicly come out and say he won't work with Winston Peters because the New Zealand First leader can't be trusted.
Bridges made it clear over the weekend that his party would not be open to forming a Government with Peters, come the election in September.
"Ultimately, I think what it does make quite clear for anyone who is in any doubt - and in a way it would be surprising if they were - a vote for New Zealand First is a vote for Labour and the Greens," Bridges told Magic Mornings with Peter Williams on Monday.
It's a question of trust - I can't trust New Zealand First and New Zealanders can't either.
Kiwis were sick of Peters' "charade of holding the country to ransom", Bridges said.
"It's clearly what works for New Zealand First. And call me old fashioned, but I think it should be about what works for New Zealanders' interests and our shared future."
He added: "Winston Peters and his team are locked at the hip with Ardern and Labour and that's the way it will be and that's the choice New Zealanders have at the next election as well. Labour, Greens, New Zealand First or National."
Bridges' position is in line with that of former Prime Minister John Key, who ruled out working with Peters and NZ First in 2005 and 2008.
Earlier, Prime Minister Jacinda Ardern spoke with The AM Show saying the decision is the prerogative of the National leader.
"Ultimately, different leaders will make different decisions," Ardern told The Am Show on Monday.
Obviously, that wasn't something that Bill English chose to make a decision on.
"All I know is that over the last two years we've demonstrated that we can work well and form a solid, stable Government with New Zealand First and with the Green Party - so people know what they get with us."
Ardern would not comment on whether she would prefer not to have to rely on Peters to form a Government.
"Voters decide that - it's MMP," she said. "I'll campaign hard for Labour to get the largest vote possible and then what voters decide is what I then work with."