The good news is that he's given measures to mark his policy by:

1. increased rentals
2. "downward pressure" on rents

The bad news is that the most easily measured one he's deliberately fudged and been very careful to never say that rents will actually go down; just that there's downward pressure on them. So he can claim when they continue to go up, that well they would have gone up more if it weren't for giving some of the wealthiest New Zealanders a tax cut.