The government's so-called 'waka-jumping' bill has passed its first reading in Parliament.
The legislation is designed to stop MPs from ditching their party and is part of the coalition deal between Labour and New Zealand First.
Under the bill, if an MP quit or was expelled from their party, they would be ejected from Parliament.
Electorate MPs could seek to win their way back in through a by-election.
Justice Minister Andrew Little said the bill ensured New Zealanders determined the shape of Parliament.
But National said it was a dark day for democracy.
"It gives party machines and leadership too much power in the Parliament," National Party leader Bill English said.