While browsing around the web, I discovered that there is a Liberal International. This is an alliance of the world's liberal parties in the same way that the Socialist International is an alliance of social democratic, socialist and labour parties (including our own, BTW).
There aren't any New Zealand members of the LI, but looking at their principles - which boil down to liberalism, human rights, and free trade and development - more than just ACT would qualify. They take a broad view of liberalism, rather than limiting it only to classical liberals, and in fact explicitly support a "social market economy" - meaning
a market that offers people real choices. This means that we want neither a market where freedom is limited by monopolies or an economy disassociated from the interests of the poor and of the community as a whole.
Which is sufficiently broad that most New Zealand political parties could sign up to it. Not that this is a problem - the LI expects a diversity of approaches on the scope of the market in each particular society. Their aim is to focus on the ideals that unite liberals rather than the details of how those ideals are implemented.