Seems fair for families, they just need to select low emission cars if they don't want to pay the tax. If they need a bigger capacity car it allows higher emission.
As for farmers no it doesn't, although you could use the highway usage for farmer 4wds and there are <7L choices.
You can see the usage up to 03 models here:
http://www.environment.gov.au/settle...ort/fuelguide/