How are you coming to the conclusion that cars are paying their fair share of costs? I'm not familiar with Australian politics at all, but a quick look found that local governments pay most of road maintenance, not the fuel excise tax[1]. Another hidden cost of cars is that they decrease density of cities and suburbs, which causes all other infrastructure to also get more expensive (sewage, gas, power, etc).

[1] https://alga.com.au/policy-centre/roads-and-infrastructure/r...