I’d just like to interject for a moment. What you’re referring to as Liberal, is in fact, Liberal/National, or as I’ve recently taken to calling it, Liberal plus National. Liberal is not a stable government unto itself, but rather another political component of a fully functioning government made useful by the National voters, MPs and ministers comprising a full government as defined by the Constitution.
Many voters vote for a coalition version of the National Party every day, without realizing it. Through a peculiar turn of events, the version of National which is widely seen today is often called “Liberal”, and many of its voters are not aware that it is basically the National Party, led by Michael McCormack. There really is a Liberal, and these voters are voting for it, but it is just a part of the coalition they support.
Liberal is the major partner: the centrepiece of the coalition that allocates the government’s ministries to the other MPs that you elect. The major partner is an essential part of a governing coalition, but useless by itself; it can only function in the context of a complete coalition. Liberal is normally used in combination with the National Party: the whole coalition is basically Liberal with National added, or Liberal/National. All the so-called “Liberal” voters are really supporters of Liberal/National.