The waka-jumping law has passed in Parliament today, but what does it mean and why does it matter?
Officially known as the Electoral (Integrity) Amendment Act, the controversial bill has become a hotly debated topic.
What is it?
If a Member of Parliament decided to leave their political party, or was forced out by the leader, they now would also lose their seat.
This means, if they were an electorate MP they would be forced to go through a by-election, but a list MP (someone voted in through the party vote) would lose the seat altogether.
The next person on the party list would take the seat.
Before, MPs could join or create another party or become an independent MP.
As an integral slice of the NZ First-Labour coalition agreement, it saw Justice Minister and Labour MP Andrew Little describe much of the commentary around the bill as "scaremongering".
He called it an "important signal" for voters that their collective vote cannot be changed by an MP that decides "on a whim they're going to do something different and distorts the proportionality of Parliament".
Labour list MP Ginny Anderson told the House in August she was elected "on the Labour Party's shoulders, I stood upon the values that the Labour Party represents".
"If I should depart from those values, I should not deserve a seat in this House."
Alternatively, the Green Party described handing over their support for it as "swallowing a dead rat", and National's Dr Nick Smith has referred to the proposed legislation as a "dog", a "crude power grab", "affront" to core Kiwi values" and a "draconian, free speech limiting" bill.
Columnist John Armstrong called it "pointless".
"The problem is that there is no problem. As a species, party hoppers may be close to extinction. There have only been four MPs who can be categorised as such since the powers in the first and so far only anti-defection law lapsed," he wrote.
A selection of 19 legal academics and political scientists made a submission to oppose the bill, calling it "undesirable and harmful".
They used the example of the four MPs who had left their party during a term and stayed in Parliament since 2005, saying it was "hardly evidence of a major threat to the integrity of the electoral system".
The bill became passed its third reading today, with Labour, NZ First and the Green Party voting for, and National and ACT voting against.