A tweak for road user charges

New Zealand's road user charge system, in which petrol excise is a level rate but diesel charges vary with the weight of the vehicle, seems likely due for a rethink in the next few years. The bigger change will be needed as electric and hybrid cars take up a greater proportion of the fleet.

But here's a little one that could be bundled in at the same time.

Currently, every diesel owner has to report mileage and pay their road user charges. Smaller vehicles have a low per-km charge; large ones have a larger charge. It would be trivially easy to impose excise on diesel equivalent to the costs that small diesel vehicles impose on roads - the same as the petrol levy. Then, exempt small diesel vehicles from having to pay road user charges while lowering the tariffs for larger vehicles' road user charges.

You'd maintain the appropriate link between vehicle weight and road user charges while saving a pile of smaller diesel vehicle owners the hassles involved with road user charges. And as more smaller vehicles switch over to diesel, it seems a simple change worth making.

Update: the comments section has helpfully pointed out the substantial issue with this scheme:

1. Because much diesel is used off road, we'd have a trade-off between the hassles of RUC for small diesel vehicles and the hassles of running an untaxed stream of diesel that could leak back into the road market. When I was a kid in Canada, petrol for on-farm use had purple dye in it, and vehicles that were not registered for farm use were forbidden from using it; they had occasional checks, and a purple tinge in your carborator could be used against you.
2. The actual hassles of paying RUC for small diesel vehicles is smaller than I had been led to believe by the diesel owner who pointed me in this direction. So it seems highly unlikely that the overall scheme is hassle reducing.

I consequently withdraw the suggestion.