2. New buildings are unoccupied, sometimes for months.
A more useful measure is rental vacancy rates. The Australian average is 2.3%, which is pretty close to "none" in practical terms.
According to the following Melbourne has a 5.9% vacancy rate. They use water usage data from the water companies for their analysis. (<50L/day = vacant property)
REIV vacancy rate is (unleased property on the books) / (property on the books).
It's always going to be an under-estimate of true vacancy due to properties that are empty full time and those that are privately managed and temporarily vacant.