Brendan Horan joins a select group of MPs who have been expelled from their caucuses as sitting members, including his now former parliamentary leader, Winston Peters.

Mr Horan's expulsion by New Zealand First after question time cuts its caucus to seven.

If the Electoral Integrity Act were still in force, he would also have been deemed to have resigned from Parliament.

The next person on New Zealand First's list, Helen Mulford, would have been invited to fill the vacancy.

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But the act, also known as the "waka-jumping" law, was in effect only between 2001 and 2005. It expired on election day 2005.

It applied to MPs who jumped parties and those who were expelled. It was resisted for some time by MPs who believed it could be a powerful and unfair weapon in the hands of party leaders.

That was the rationale behind the sunset clause.

Mr Peters tried to get the law resurrected and Labour was required to support its reintroduction as part of his confidence and supply agreement in 2005 but it did not go further.

The law was passed after the defection of Alamein Kopu from the Alliance in 1997, mass defections from New Zealand First in 1998 when its coalition with National blew apart, and earlier defections from Labour and National to form United New Zealand in 1995.