> Those taxes don't actually cover the cost of maintaining the road system

Yes they almost did.

Only a few years ago the National Land Transport Fund (NLTF) was almost entirely self-sustaining, funded by road users.

Recently Crown funding (grants and loans) expanded significantly to ~40% of fund income.

But approximately 30% of government transport spending is being spent on rail (to placate voters I think). Before the Land Transport (Rail) Legislation Act 2020, not much we spent by the NLTF on rail.

Currently ~3% of driven kilometres by car use electricity - so as that number increases, BEV and PHEV vehicles will need to have increased taxation. Presumably something like $700 per annum (currently about how much a person driving a petrol car pays on excise tax).

Ultimately it is almost tautological that road users pay for roads, since government spending comes from taxes, and most people use cars. How things get earmarked is just sophisticated accounting.