Owners who leave properties vacant will be slugged with a new tax under a Victorian government push to free up more housing for sale and reduce rents.

The new vacant residential property tax is expected to raise about $80 million over four years, coming into force on January 1.

The tax is among a suite of changes the government has announced to make housing more affordable, including scrapping stamp duty for first home buyers on properties worth up to $600,000.

The government will launch a $50 million program to co-purchase properties with first home buyers in which the government would retain an equity share.