A total of 82,724 properties in Melbourne are unoccupied, or 4.8% of Melbourne's total housing stock, according to Prosper Australia's 2015 Speculative Vacancies Report. The number is a 28% increase on last year's result.

Up to 18.9% of Melbourne's investor-owned properties are vacant. 

The annual report examines water consumption over a 12-month period. Properties with low water consumption, measured as equivalent to or less than a dripping tap, are considered to be vacant.

In 2015, a record number of apartments have been approved for construction in Melbourne, often on the basis their construction will contribute to increased stock. However, the report raises concerns the new apartments will do little to increase stock, and therefore won't alleviate housing affordability pressures.

Karl Fitzgerald, Prosper Australia's Project Director, said, "The incentive for property speculators to hold prime locations empty is an affront to anyone locked out of housing. The findings prove we do not have a housing supply crisis, we are literally locked out."

Prosper is advocating for higher land taxes, which would increase the cost of holding property, and an end to stamp duty, which it says impedes property transactions.

"The challenge is to address the disconnect between supply and occupancy," said Fitzgerald. 

It is unclear how many of the vacant properties are owned by foreigners. However, many of the vacant apartments were sold off-the-plan, a popular method of purchase for offshore investors.

The Speculative Vacancies Report is the only study of property vacancy in Australia.