I don't even think we'd need to go that far. Also there is a place in the market for rentals, short and long term, its just taking up too much of the market. Therefore it just becomes a matter of the government augmenting the market.
Increase the comparative costs of these activities by minimising the existing incentive structures and people investors will self select out. The rental/investment property market becomes smaller, and a large vector of demand eases upward price pressure on housing.
The tricky part is, as soon as a government starts hitting investors, they'll complain and hit back, then if the rental market is squeezed that'll have an electoral effect also. This is why i'm thinking the HAFF from the federal government is maybe a good place to start the process. Because you, hopefully, remove a whole section of the lowest, most at risk people in the housing investment sector, nullifying the most poignant attacks if other policies were to come in.