Historically Diesel was alot cheaper than Petrol in Oz. Diesel is afterall cheaper to produce.
Sometime in the 90's I think it has become more expensive here. I believe this is due to Asian demand and also govt taxing in part.
Diesel engines are more efficient than petrol/ULP and this has contributed to more diesel powered vehicles being demanded.
The Oil Companies worked out also that they could charge more due to a larger no. of cars running on diesel (in some countries eg UK diesel is still alot cheaper.
Diesel pricing does have a seasonal factor also - due to demand in the Nthrn Hemisphere and its relationship with Heating Oil.
