The point of negative gearing is to encourage investments in...

The point of negative gearing is to encourage investments in rental accommodation so renters have somewhere to live. It doesn't matter whether the home is new or old. Besides which in the majority of cases why would an investor want to provide a new home to tenants? As new homes are generally more expensive and you don't get that much better rental return. The only thing new homes have is you have more depreciation deductions and in theory less maintenance costs in the early years.