Well, I would make the case that Labour was the party that first agreed to cut the top tax rate.
In their 2005 agreement with United Future, it was agreed:
they will conduct a review of the current business taxation regimes with the view of ensuring the system works to give better incentives for productivity gains and improved competitiveness with Australia.
This is political speak for “we’ll reduce the business tax rates to match Australia”.
Both Labour and United Future knew full well that this would end up bringing down the top personal tax rates too. I know this because I heard it explained as such by a United Future MP. You simply can’t have company rates lower than personal rates, because it opens a loophole for people to decrease their tax liability.
Had Labour been in power this term, there can be little doubt that they would be doing the same as National. Lucky for them, they’re not so they can now oppose the change that they started.