The Green Party would need the support of at least 75 per cent of its party members to expel an MP under a new waka-jumping bill that was introduced to Parliament yesterday.

The Electoral (Integrity) Amendment Bill would ensure a party's representation in Parliament remains in proportion to its share of the party vote if an MP resigns, is expelled, or decides to jump ship to another party.

The bill is part of the Labour-NZ First coalition agreement and its introduction was added to the Government's 100-day programme. The bill is expected to pass with the support of Labour, NZ First and Greens.

The Greens successfully lobbied for an extra safeguard, which is the only notable difference to the 2001 law - any decision to expel an MP from a party would have to comply with the rules of that party.

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For the Greens, that means the expulsion would have to be endorsed by a consensus of its party members and, if consensus is not reached, it would need the support of 75 per cent of members in a vote.

Justice Minister Andrew Little said the additional rule increases the threshold for a party to kick out one of its own MPs.

The other rules mirror the previous law. At least two-thirds of a party caucus would have to agree with the party leader to expel an MP, after written notice to the MP and a period of 21 working days for the MP to respond.

Little said the 2005 Supreme Court decision between the Act Party and former MP Donna Awatere Huata also was a reason to include the additional clause about party rule compliance.

The court ruled that Huata should be removed from Parliament and replaced by an Act list MP, rather than stay on as an independent.

Little said the court had made it clear that it did not want to have to make qualitative judgments on whether expulsion is justified, which could lead to interviewing MPs.

The court should instead be able to see "on-the-surface compliance" by looking at the party rules.

"They don't want to go beyond that. Putting in this added thing about compliance with the party rules is an extra, easy requirement for a court to look at, if there is a challenge to a decision."

A party-hopping law came in under the Labour-Alliance coalition in 2001, but had a sunset clause and expired in September 2005. It was re-introduced two months later as part of the Labour-NZ First confidence and supply agreement, but languished on the order paper until National won the 2008 election - when it was dropped altogether.

At the time, the Greens voted against it, with then-co-leader Rod Donald saying the bill was "anti-democratic and draconian".

"You've got to ask why people in Labour and NZ First think some of their MPs are going to desert their parties," Donald said.

The law required MPs who wanted to leave their party or were expelled, to resign from Parliament. It followed the case of Alamein Kopu, who left the Alliance in 1997 but stayed in Parliament and voted with National, and the public fracturing of NZ First, which lost nine MPs in 1998.

Last month it was revealed that the Greens were considering asking for policy concessions ​in exchange for its support for a waka-jumping bill, but faced criticism from Labour and NZ First after the internal proposal was accidentally sent to media and made public.

Shaw later said that the Greens had ditched the horse-trading idea and would support the bill, though in its internal memo the Greens noted that doing so had the potential to upset its core party members.