Interesting that they are even thinking of this.
I am pretty sure that this will only serve to redistribute the wealth up again.
If you raise GST, that raises the amount of tax paid on all purchases, those at the top have creative ways of getting GST right offs whilst those at bottom end up footing the bill.
We already know that NZ'ers spend outside their means, by lowering income tax, you will effectively help out the top more than you will help out the bottom.
Those at the top will benefit most from income tax breaks whilst those at the bottom on a proportional scale may benefit the same amount however the fact that they will be paying 2.5%-7.5% more on everything they do in their lives will be a substantial problem.
Add to that the fact that people who are struggling to make ends meet are likely to borrow money to survive and have to end up paying more on services, they will end up having a compounded effect whereas those at the the top will be happy
National is known for widening the gap between rich and poor though, so it really comes as no surprise.