NZ First MPs who are sacked or resign from the party could face a $300,000 fine if they don't give up their seat within three days, though legal experts say it's unlikely to be enforceable.
The rule was written into NZ First's constitution after Brendan Horan was sacked by party leader Winston Peters in 2012. Horan, who has remained in Parliament as an independent MP, has dubbed it "Winston ruling by decree".
"That's bullying and intimidation of the worst sort and it's another way to make sure MPs stick in line with whatever the leader said. It's just about giving Winston Peters more power," he said.
Horan was expelled after allegations he took money from his late mother's accounts, though he has strongly rejected the allegations.
MPs were elected to represent voters and should remain there until they were voted out, he said.
Party president Anne Martin did not return calls but in a statement said the provision was a response to "the problem of MPs who resign from the party or caucus or are expelled, and who stay on in Parliament as an independent or 'waka jump' to another party".
"Clearly a list MP is elected as a party representative and it is contrary to the scheme of New Zealand's MMP Parliament that such a situation should be allowed to continue. Party members wanted something done."
The clause was added last year but was yet to be ratified, she said.
Professor Andrew Geddis, a constitutional law expert at Otago University, said the clause was unenforceable.
"What they're trying to do is essentially strongarm someone out of Parliament through this fear of a financial penalty."
There was no law preventing an MP from switching parties during a parliamentary term, with the Electoral Integrity Act expiring in 2005, and this was NZ First's attempt to create a restriction.
Geddis said Peters had successfully challenged a similar situation in 1993 when the National Party tried to make him sign a clause preventing him from standing on his own or for another party in Tauranga when he failed to win its nomination.
"The High Court said, no, we won't enforce that on public policy grounds because the question of who can and can't be elected a member of Parliament, that's too important to be decided by an employment contract between individuals."
Public law expert Dr Matthew Palmer also said it was probably unenforceable because when an MP resigned or was sacked by the party they were no longer a member and not bound by that constitution.
Meanwhile, a 2012 rule which required all MPs except for the party's leader to pay 10 per cent of their salary to the party's election campaign has been axed.
- The Dominion Post