It is easy to identify the problem; finding the right solution is another matter altogether.

That is how the Prime Minister yesterday summed up the frustration of many MPs that a list member could be expelled from his caucus yet stubbornly still remain in Parliament as an independent MP.

John Key is right. There is general agreement across Parliament that - presuming Winston Peters is justified in dumping Brendan Horan - the latter should resign from Parliament.

One answer is to resurrect the long-expired law stopping MPs from jumping ship to other parties. The so-called "waka jumping" legislation passed by the Labour-Alliance coalition in 2001 did its job. But it had a "sunset clause" and expired in 2005.

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On the urging of NZ First, Labour tried to revive the law late that year. But the measure eventually foundered for lack of numbers. National and the Greens opposed it.

The Prime Minister zeroed in on the essential dilemma: how do you write such a law to ensure the party leader could not use it to purge his or her caucus of malcontents or rivals for the top job.

Labour's failed bill stipulated that the leader would need the support of two-thirds of the caucus for any expulsion.

But the Greens argued that if MPs wished to oppose an expulsion, that meant they would effectively have to be prepared to roll the leader.

Much of MPs' unhappiness with Horan flows from him still picking up a big salary plus numerous expenses and other perks, thus reflecting badly on Parliament as a whole.

However, waka-jumper legislation is not the right answer. The democratic rights of genuine caucus rebels should not be sacrificed for the sake of expediency.

Better to do nothing. As Bill English noted back in 2005 during debate on Labour's second waka-jumping measure, any MP who became an independent was thrown out soon enough - by the voters.

"Those people are never seen again. That is pretty harsh punishment. They get dealt to."

That is something Horan will have to ponder over the holiday break.