So much of the wealth of our middle- and upper-class is dependent on property ownership and rent-seeking, it's depressing. That population essentially needs to vote against their own self-interest to help improve housing affordability, so it's hard to see that ever happening. The best I could foresee is a government forecasting a stepped reduction of relevant tax benefits over time (e.g. in three years negative gearing gets reduced by half, then half again the following year, etc.) and then future governments honouring that commitment. As you pointed out though, it's a surefire way for any Australian political party to shoot themselves in the face.
I sometimes wonder how strong the demand needs to get for more-affordable housing before the market responds enough to matter. State and local govt could likely have a role in unlocking infill developments and increasing the allowed densities, but I'm not plugged into the planning system. I also strongly agree that state government should be more proactive as a housing supplier (in conjunction with private industry).