Let the drivers report the odometer reading, and add the amount for the last period to the new cost.
You'd get some folks who lie, but there's already inspections in place for sales in some states. If the owner wants to sell the vehicle, require an odometer check and a final payment.
Basically - I don't really think there's a ton of extra admin cost here. Most folks will self report just fine.
Plus, we should be encouraging emissions inspections anyway. At least in the US many states require yearly or bi-yearly inspections to confirm nothing is wrong with the emissions systems in the vehicle. Good way to encourage basic maintenance and to vet manufacturer emission claims against real world data.
It's tied to yearly vehicle registration there too, and includes an odometer reading.
This is just not realistic to apply, and especially in Australia. You'd need someone to actually record those numbers, which would be a massive overhead of salaries on its own. Then you have crazy distances... you'd either send the inspector to that single farm 300km from the nearest town, or force people running it to being the trucks in for a check monthly.