"If National were pushing for first past the post to give them a better chance to govern, isn't Labour opposing it because it doesn't want National to have a better chance to govern? "

Isn't the point that in a democracy the party with more votes should have the first opportunity to form a government? Under FPP National was able to continue to govern despite getting less votes than Labour in both the 1978 and 1981 general elections. While proportional systems have their drawbacks they do tend to deliver government's led by the party more voters prefer.