In Austria D isn't really much. Those are basically uninsulated buildings from the 50's. That's a really low bar and I don't know what building can be worse than D. A garden shed maybe? Making D mandatory by law seems like pointless political posturing. But then again I don't know buildings in the Netherlands.

It's an European standard, so maybe the criteria are different in Australia? My current apartment has an E rating and it has double paned windows everywhere, floor + ceiling insulation and I have neighbours on all side including above and below, meaning not much energy is transferred to outside.

Edit: just noticed I misread Austria as Australia (lol), point still stands though, a D rating is not that bad.

I'm not sure how the Austrian system works or what homes are like there or in the Netherlands, but a lot of older homes in the UK can apparently only reach a D even after all the standard insulation retrofits are done. This is especially a problem in areas of the UK that are poorer already.

In the UK it's E