Alexei of Thinking About Tomorrow has done a quick analysis of party leader's history on conscience votes. The result is a tree which measures the effective difference between people's positions - and its rather surprising, in that it places self-declared "social liberal" Don Brash out on the fringe with Winston Peters (who along with Lockwood Smith voted against Homosexual Law Reform) and Tariana Turia. I guess that's what you get for selling your soul to pander to bigots.
Meanwhile, the other party leaders (and John Key) form a fairly loose cluster at the extreme opposite end of the spectrum. Sure, there are differences - but none so great as those with the other group.
You can see the full picture here.