Campaigning has begun already for a byelection scheduled for March 28 to replace Northland MP Mike Sabin.

The National MP, who was just a few months into his second term, resigned on Friday citing "personal reasons best dealt with outside Parliament".

Neither he nor Prime Minister John Key would make further comment on his departure but Mr Key revealed yesterday that a byelection - expected to cost taxpayers up to $1 million - would be held on March 28.

Some of National's opponents are not wasting any time with Labour hopeful Willow-Jean Prime starting her campaign on Saturday, the day after Mr Sabin's resignation - despite having her first child a week earlier.

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The lawyer and Far North District councillor spent the morning at Kawakawa's St John market promoting the byelection, signing up new members and enrolling voters. Labour has yet to officially name its candidate but Mrs Prime said she was keen to stand. If chosen it will be her third election in 18 months.

National's current Northland electorate chairwoman, Rose May, said nominations for a new candidate opened yesterday and will close at noon on February 12. The electorate committee would whittle the nominees down to a shortlist of five, with the party's 100-plus delegates expected to make their choice at Kaikohe's Memorial Hall on February 28.

Mrs Rose said it was the same process as used in 2011 but in a shorter timeframe. Anyone who lived in the electorate and had been a party member for at least the past year was eligible for nomination.

She was "absolutely" confident National could retain the seat, which was won last year by Mr Sabin with a 9300-vote majority.

Mr Key said the March 28 byelection date gave parties sufficient time to prepare while allowing the process to move ahead.

However, blogger Cameron Slater, who has close links to National, announced the byelection date on his Whaleoil blog a day before Mr Key's announcement.

Until a new MP is elected Mr Sabin's departure leaves National with 59 seats in a 121-seat Parliament, making the party more dependent on support parties to pass legislation. The smaller parties have yet to decide whether to join the byelection fray.

The Greens' David Clendon, the third highest polling candidate in 2014, said it would be up to the party executive whether to contest the byelection.

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Key: I didn't know until late last year

Prime Minister John Key says he only learned of the problems facing Northland MP Mike Sabin late last year.

Mr Key had been under pressure to reveal how long he had known about the possibility of Mr Sabin's resignation, given that the by-election is expected to cost taxpayers about $900,000.

Rumours had been swirling around the MP for some time but Mr Key said yesterday he only learned Mr Sabin was facing "personal and family matters" in the last week of the parliamentary term last year. He rejected suggestions that he knew of the issues plaguing Mr Sabin before the September general election.

Mr Key said he found out Mr Sabin was planning to resign "one to two days" before it happened. Going into the election, and until Friday's resignation, Mr Key said he had "enormous confidence" in Mr Sabin as an MP. He had a good grasp of policy and had the potential to be a future Cabinet minister.