Treasurer Scott Morrison has linked housing affordability to Australia's birth rate, suggesting would-be home buyers are delaying having kids so they can get into the property market.

But Mr Morrison has - once again - poured cold water on the idea that axing negative gearing tax concessions is the way to lower barriers to entry into the property market.

He also rejected suggestions investors are causing a property bubble, just days after new data showed the share of loans taken out by investors had climbed back above 50 per cent for the first time since the prudential regulator took action in 2015 to curb bank lending to would-be landlords.

Instead, Mr Morrison blamed a lack of supply for new housing for surging house prices in Sydney and Melbourne.