The system results more complicated than the British or the Spanish ones, which rely exclusively on CO2 emissions: Cars emitting under 140 CO2 g/km get a "bonus" or discount from the car tax (300 EUR), but you can gain an additional bonus if you use "alternative fuels and technologies" (500 EUR) or a Diesel Particulate Filter (200 EUR). The malus system starts when your car pollutes more than 180 g/km: 25 EUR for each additional gram (initial plans started the malus at 160). If you thought this was complex, there's more: total bonus cannot result in more than a 500 EUR discount and the final car tax can't be negative, although it can be offset.
Criticism has spread considerably in Austria because a family carrier like a Volkswagen Touran with a 1.4 TFSI engine ends up paying more taxes than a Lexus Hybrid.
[Source: Der Standard (link is in German)]