New Zealand First leader Winston Peters says a new plan to fine its MPs if they leave the party but stay on in Parliament will help ensure fairness for voters.

The party is changing its constitution to stop its MPs staying on in Parliament after they have resigned or are expelled from caucus.

The new constitution states that any new member who agrees to become a New Zealand First candidate will have to sign what's intended to be a legally enforceable contract.

That stipulates that if they resign from, or are expelled from, the New Zealand First parliamentary caucus, they must quit their seat in Parliament within three days. If that contract is broken, the new rules say the member would be liable for $300,000 in damages.

Mr Peters told Radio New Zealand's Morning Report programme that under the MMP proportional system, the ratio of any party voters elect should remain the same, and that would be upset if a list MP defected.

"If they could no longer be with that party but wanted to stay on as an MP what authority or what mandate have they got? The answer is, no mandate at all."

Photo: RNZ / Demelza Leslie

It is generally up to MPs themselves to decide whether or not they will quit Parliament altogether once they have become an independent, apart from specific circumstances laid out in the Electoral Act.

The law says an MP can lose their seat if they are convicted of an offence punishable by imprisonment for life or by two or more years' imprisonment, death, mental or physical incapacity, or absence from New Zealand when his or her services are required.

Horan sparked review

The changes follow the expulsion of Brendan Horan in 2012 after accusations that he stole money from his late mother.

Photo: RNZ

There was much political debate at the time about whether a list MP had a mandate to stay on once they're no longer part of the caucus, as opposed to an electorate MP.

Mr Peters said at the time that Mr Horan was honour-bound to leave Parliament altogether.

There have been other cases in the past, where list MPs, who have either been expelled from or quit their caucus, have then served out the term as an independent. That includes the former United Future MP, Gordon Copeland and Donna Awatere Huata from ACT.

However, it remains questionable whether such a contract would be enforceable.

Public law professor Andrew Geddis believes the courts would throw out the contract on public policy grounds because it effectively amounts to New Zealand First twisting a member's arm to quit Parliament.

He said no court would put an MP in that position - of having to choose between losing their job as an elected Member of Parliament or paying what could potentially be a crippling $300,00 penalty to stay there.

Mr Geddis also said the courts would not want to be seen to be interfering with the membership of the House of Representatives.

While it appears those changes are unlikely to be enforceable, nonetheless, it is still an extraordinary step for a political party to take, and would make it much harder for New Zealand First to attract willing candidates.

Donation rule removed

In the 2012 constitution there was also a requirement that each party MP, except the leader, should donate 10 percent of their base parliamentary salary to the party, to help fund the election campaign.

That clause has been removed from the 2013 version, which has been posted by the Electoral Commission as the party's new, official rules.