New Zealand first leader Winston Peters is picking the Prime Minister will help his Northland by-election chances, not harm them.

John Key will be campaigning in the electorate today, immediately after his visit to Korea and Japan.

And it follows an opinion poll showing National's well behind in the by-election race.

Mr Peters thinks the more people see of National in Northland the better it will be for him.

"Because they know they've never seen him here before, they've never seen half these cabinet ministers, never seen half these backbenchers before, and they won't see them again if they vote for National."

The National party's campaign chair, Cabinet Minister Steven Joyce, admits they are behind and attributes some of it to their candidate being a relatively unknown.

"It's been a tougher challenge than we expected, no doubt about that."

Steven Joyce has faced questions as to if National set too short a campaign period.

"Don't forget when all this timetable was set, Winston wasn't standing. So in terms of the comparison between someone who's been around 40 years and someone who's been around 5 minutes, it wasn't as clear at that point."

Opposition parties are scenting blood in the water.

Labour MP Kelvin Davis says the Government is in a bit of a shambles.

"They've botched up the by-election in an electorate that they've held for decades, it is slipping through their grasp as we speak."