Firstly, if you think investors are holding the market up in only the areas you cover. Then what is holding prices up in the areas you don't cover? (pretty much the rest of Australia, even very low yielding areas)
Please don't run the old 'immigration' line. That doesn't hold much water anymore, at least outside of Sydney.
7% gross yields? I'm sceptical. I think you'd be lucky to see over 5%. Even in bottom of the barrel places like Gorokan, Mt Druitt etc
And its pretty much 2% expenses above interest rates.