Treasurer Scott Morrison has indicated a tax on empty housing stock could be in the May budget, along with plans to encourage elderly Australians to downsize their homes.

He also left the door open to allowing first home buyers to raid their superannuation to fund a deposit, and said further tightening on regulations for foreign investors had "his very close attention".

Speaking ahead of an agenda-setting address on housing affordability on Monday, Mr Morrison ruled out changes to negative gearing and labelled Labor's plans to cut negative gearing "cruel".

"Negative gearing has been around for a century. It's an established structural component of Australia's housing market; if they are going to pull the rug out, who is going to put a roof over people's head," he told ABC Radio.