I don't know what fuel prices are like in Australia, but here in Europe I pay around €0.05/km to fuel my ICE car. Taxes make up around 70% of that, so an EV tax of €0.015/km (1 AUD = 0.6 EUR) wouldn't be unreasonable once everyone is driving EVs.
As you say there's probably a better way to tax it though (it doesn't make sense someone with a Tesla Model X pays the same as someone with a Renault Zoe). If you drive 100,000km over the lifetime of your car (this is just a nice round number for examples sake) that's only €1500 in tax revenue. Higher taxes on the sale of large and luxury cars could generate the same revenue with little impact on the overall price (if you are already planning to spend €50k+ on a new car).
As you say there's probably a better way to tax it though (it doesn't make sense someone with a Tesla Model X pays the same as someone with a Renault Zoe). If you drive 100,000km over the lifetime of your car (this is just a nice round number for examples sake) that's only €1500 in tax revenue. Higher taxes on the sale of large and luxury cars could generate the same revenue with little impact on the overall price (if you are already planning to spend €50k+ on a new car).