Maybe diesel costs less to refine? At least that used to be the case unless the refining process changed.
As for car tax, Australia is on the same boat as Portugal with a higher cost than average. Maybe Portugal dealerships are a bit more willing to give you a deal to offset the higher price?
They just better act quick before law makers make it even tougher or discouraging to own a diesel. Or at least slow the release of diesel vehicles so the impact isn't nearly as much as it could be. Might be why Toyota has an aggressive hybrid strategy in place.