Don't underestimate the pollie trick either. They will increase migration to fill the empty units. If someone buy the new apartment they must have moved out of their old accommodation to move in or rent it out. So where are the tenant unless they bring in new migrants.
Check out the completed apartment block on the river side opposite Canterbury train station in Sydney. About 10 units for sale and lease and the thing is to look at its surrounding neighborhood.....there are around another 10 high rise apartment block under construction or about to complete. And that's number is just within 500 metre radius of this apartment.
