Gillard is also heavy supported by the Labor right, which would be more inclined to favour a status quo on marriage rights.
As for Abbott, I doubt he'll allow a conscious vote before the next election. Politically he has nothing to gain from one. His current stand saw little movement in the polls last time there was a vote and given the huge margin the Coalition has they can take a hit if it comes. Not to mention that it would deny the Labor government something new to their "legacy".
Anyway, good on the NZ government for doing the right thing.