There are a few other cases where it applies as well, but you're right that it doesn't apply to investment properties.

Also, it's no longer always a full write off, it's a write off at a capped rate. Those with low incomes have a full write off, those with high incomes see a more limited effect.

Note to self : PPOR means 'Principal Place of Residence' here.

Since most people would like themselves a PPOR that still has the effect of heating up the market.