Imo its a combination of land restrictions,regulation or red tape and incentives to borrow for property,and possibly foreign buyers but we don't know because we don't keep records.
I would like to have read stephen bartholomeusz business spectator as imo his comments on all subjects seem very realistic,but so far i have not subscribed. From home page 'scaling back a large skew towards housing'. 'there is a case for some tinkering with a system that incentivises borrowing for property'.
