The waka-jumping bill is consistent with The Bill of Rights Act, despite the "chilling effect" it could have on an MP's freedom of expression, Attorney-General David Parker says.
But Parker says the bill's aim to maintain the proportionality of Parliament, as determined by voters, outweighs the impairment on an MP's right to freedom of expression and freedom of association.
Parker has signed off the Electoral Integrity Amendment Bill - commonly referred to as the waka-jumping bill - as consistent with the Bill of Rights Act, despite conceding the negative impact on MPs' rights.
The bill is currently before select committee, though it is not certain to pass into law because the Green Party, which has traditionally opposed such legislation, has not pledged its support beyond the select committee stage.
The bill would mean that if an MP left their respective party, or was expelled, they could not switch parties or become independent MPs but would be removed from Parliament.
The National Party has criticised the bill as giving too much power to a party and its leader, who would be able to whip MPs into line with the threat of expulsion.
The safeguards against unjustified expulsion include that a party leader must "reasonably" believe that an MP is about to distort parliamentary proportionality, the MP must be given 21 working days to respond, and two-thirds of the party caucus must agree.
The process must also comply with the party's rules; for the Greens, that means an 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.
In his Bill of Rights Act assessment of the bill, Parker said the bill could leave MPs fearful of expressing a dissenting view.
"The proposed legislation raises a prima facie limit on a member's right to freedom of expression because the prospect of facing an enforced departure from Parliament will have a chilling effect on the expression of dissenting views by MPs," Parker wrote.
He added that the bill is intended to impact MPs who have acted inconsistently with their party, "but it has the capacity to suppress the expression of dissent by MPs who have not passed that threshold, but fear they may be perceived to have done so".
"The suppression of dissenting voices, whether intended or not and regardless of the circumstances, is a solemn matter for any Parliament. Freedom of expression in the House has a special constitutional value."
But he conceded that the rights to free expression and association are necessarily impaired by the need to maintain parliamentary proportionality, as determined by voters.
"The impairment of the rights is significant, but there appears to be no alternative way to restore the proportionality of political party representation in Parliament, other than by removing the member who has distorted it.
"If they remain in the House, they will continue to cause the distortion. I am therefore satisfied that the impairment is minimal."
The bill is part of Labour's coalition agreement with NZ First. The Greens supported the bill to select committee, but co-leader James Shaw has said the party's ongoing support is not guaranteed.
The Greens have voted against previous versions of the bill, with former leader Rod Donald calling the 2001 bill the most "draconian, obnoxious, anti-democratic, insulting piece of legislation ever inflicted on this parliament".
Former Green MP Sue Bradford said the Green's support of the bill was dishonourable, while former MP Keith Locke has also criticised the party for voting for it at first reading.