Taxing Victorian homes left vacant for 12 months or more would make housing more affordable and raise money for social housing programs, a charity group says.

Prosper Australia's 2015 Speculative Vacancies report, which looked at water usage, suggested that almost 25,000 dwellings were demonstrably unoccupied across Melbourne in 2014.

Launch Housing chief executive Tony Keenan said there was a housing crisis in Melbourne and those unoccupied homes should be available on the rental market.

"It's a tax that no-one ever has to pay. The simple way not to pay it is to rent the properties out," he told 774 ABC Melbourne.

"That would be a big injection in housing. We'd probably see a modest drop in rentals and it would certainly make those houses available."

Mr Keenan said any proceeds from a vacancy tax could be used to fund social housing initiatives.

He said Launch Housing's South Melbourne service was working with a motel "full of families and kids" who had escaped family violence.

"None of those kids are in school. We're working to get them into local schools — we've got volunteers who work to try and help them catch up with their schoolwork," he said.

"But the social harm that's caused by not being able to house families quickly is immense.

"We think it is quite reasonable to say:' look, if you've got an investment property, rent it. If not, contribute to the social harm you are causing by not renting it'."

Similar schemes are used in other jurisdictions.

In England, councils can charge extra tax on properties that have been left empty for two or more years.

Various exemptions are available, for instance for houses undergoing renovations.