It is unfortunate the new Government feels the need to introduce a law against "waka jumping". Political parties ought not need legislation to keep their MPs loyal. And a party should not have the power to expel a member from Parliament if the individual disagrees with the party line.
Party discipline is already stronger in New Zealand than in comparable representative democracies. Westminster often hears members criticise their own party's position and vote against it. A Republican majority in the United States Senate does not guarantee passage for bills promoted by the Trump Administration, fortunately.
While it has been rare for New Zealand MPs to "cross the floor" on a particular issue, it used to be permitted when someone felt strongly enough and the Government's whips could preserve its majority. It is only since the adoption of proportional representation that one or two small parties have needed a law to preserve their voting numbers and it does them no credit.
The phrase "waka jumping" was coined during the first MMP coalition when Winston Peters was expelled from its cabinet and his party split, with nine MPs continuing to support National, which also obtained the vote of a deserter from the Alliance Party in Opposition. The next coalition, of Labour and the Alliance, enacted a law requiring members to resign from Parliament if they would not vote with the party under which they were elected. It did not stop the Alliance rupturing within three years.
The law had a sunset clause and duly expired at the end of Labour's second term. Its third-term partner, NZ First, had it reintroduced but the bill was still awaiting passage when the government changed in 2008 and it was dropped.