The Green Party has left the door open to withdrawing its support for the Government's waka-jumping bill, though co-leader James Shaw does not anticipate that happening.

The Electoral (Integrity) Amendment Bill is part of the Labour-NZ First coalition agreement and will pass its first reading today with the support of Labour, NZ First and Greens.

It 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 Greens have traditionally opposed waka-jumping legislation, but Shaw said extra safeguards had been added to the bill to curb the power it gives to a party leader.

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Today he said there was a "thin chance" that the Greens would pull its support for the bill after the first reading, but he was "relaxed" about the current state of the bill.

"That possibility always exists. We've got to get [it] to select committee and see what it says."

With National opposing the bill, it will need the support of the Greens to become law.

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

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.

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.

NZ First leader Winston Peters said the bill was "very important".

"Proportionality is critical, and we want that honoured. You can't have people destroying proportionality."

Former MP Gordon Copeland, who was at Parliament today to speak at a Christian rally about the parliamentary prayer, said he was glad there was no waka-jumping law when he left United Future and became an independent in 2007.

Copeland resigned over the party's support for the anti-smacking law, which he said was against the party's principles.

He said list MPs should be able to leave a party and become independent.

"If the party itself deviates from its own policies, as what happened in our case with Peter Dunne deciding to do things differently after the election, I thought I was staying in the party by leaving the party, in order to support the values under which we were elected.

"Over my time in Parliament, there were a number of MPs who voted against their conscience because they were ordered to do so by the Prime Minister or the Leader of the Opposition."

Justice Minister Andrew Little has said the 2005 Supreme Court decision between the Act Party and former MP Donna Awatere Huata also was a reason to include the additional safeguard 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 party rules.

The Greens had previously considered asking for policy concessions ‚Äčin exchange for its support, 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.