It’s an interesting quirk of our Parliamentary system that it’s completely up to each party how they determine their leaders.
The Green Party, for instance, gives members the right to determine its leaders, other than stipulating that one must be female and one male.
Labour recently reformed their own rules so that a vote is held, and that vote is weighted 1/3 to its current caucus of MPs, 1/3 to direct party members, and 1/3 to indirect party members through affiliated organisations. (ie. unions that back Labour) That’s not perfect, but at least members get their say, even if Caucus gets more of a say than everyone else.
It’s only the parties on the centre-right side of politics whose leaders are determined with no real nod to democracy, and no need to show that they can actually command popular support before they are appointed. Notably, New Zealand First and the National Party both stipulate that their caucuses in Parliament are the only people who get a say in who the party’s leader is. (Must have been awkward for NZ First if anyone asked about it while they weren’t in Parliament…)
It’s not really acceptable to have the Prime Minister being chosen purely by party insiders with no real democratic accountability until 2017. This was the sort of thing people voted against when they decided to bring in MMP, that we’ve had enough of smoky back rooms.
Instead we will get a PM determined by as few as 28 people, because there’s only 59 votes that count in determining who leads the National Party. Why anyone would pay to be a member of a club that cares nothing about their opinion is beyond me.