The National Party says the waka-jumping bill introduced to Parliament last week would gag MPs from expressing opposing views for fear they will be expelled from their party.

And National's justice spokeswoman Amy Adams says the bill would make electorate MPs more loyal to party leaders than to voters due to the power that leaders would have to expel an MP.

The Electoral (Integrity) Amendment Bill was introduced last week and is part of the Labour-NZ First coalition agreement and the Government's 100-day plan. It should pass with the expected support of Labour, NZ First and the 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.

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.

The Greens successfully lobbied for an extra safeguard - 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.

Amy Adams said today that National would oppose the bill.

"The bill would effectively prevent individual MPs from speaking out on points of principle and policy, and ensuring the voices of their communities are heard. Worse still, it would enable party leaders to advise the Speaker that a caucus member isn't acting as the leader would want and then move to force that member out of Parliament.

"This makes individual MPs more answerable to their party leader than to the voters that elected them. Allowing party leaders to overrule the wishes of voters is fundamentally wrong."

Adams said the bill was about keeping any factions within Labour, NZ First and the Greens in line, and should be called the Winston Peters Self Preservation Bill.

"The reason the coalition wants to push this piece of legislation through as one of their first bills is to ensure unhappy MPs don't jump ship."

Justice Minister Andrew Little said last week that there was a high threshold to expel an MP, and the rule about compliance with party rules would increase that threshold.

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".

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.