Prime Minister Bill English says Shane Jones will not be a threat to National MP Shane Reti in Whangarei.
"They'll vote for the guy who's got his feet on the street doing the hard work," English said.
"For a local MP, people vote on how hard the local MP works, how committed they are to the community.
Jones, a former Labour Cabinet minister who stood for the Labour leadership in 2013, is expected to be announced on Friday as New Zealand First's Whangarei candidate.
English said Reti was a very good local MP.
"He will win the seat by a long way."
English said Jones knew he had made the wrong choice in standing for the Labour Party.
"It is much the same for a lot of competent politicians."
Jones was elected to Parliament in 2005 on the Labour list.
He was recruited by the National Government in 2014 to become a Pacific Economic Ambassador, a job he finished only last month.
Reti, a doctor, returned from overseas to contest the seat after Phil Heatley retired in 2014.
Reti said he welcomed the contest and that "every candidate has my respect."
His majority in 2014 was 13,169.
He made much of him being a local - Jones is from Kerikeri.
"All my children were born in Whangarei, all my children went to local schools," said Reti.
"I have my sweat with the city for 20 years, delivered dozens of children there. I'm known. "I'm the local so people will be deciding whether that local voice is important to carry to Wellington."