The short answer to your question is that if a member chooses to leave the party after having been elected from the list—or from the electorate, for that matter—they remain a member of Parliament. There's no legal consequence, but, of course, there are political consequences. The New Zealand experience is that the electorate takes a very dim view of people leaving their party without good reason. Those members who do tend to be punished at the next election.
On July 26th, 2016. See this statement in context.