that would be a carry over of the European registration system, thus the aircraft in theory would be capable of much more, potentially 544 or 600 mtow.
Not quite correct:
European countries certify their aircraft to an European ultralight standard of 450 kg or 472.5 kg (the 22.5 kg is the addition for a parachute recovery system). If imported into Australia and registered with an RAAO, that organisation has no choice but to limit the aircraft to 450 kg/472.5 kg MTOW even though the class regulatory limit might be 600 kg.
So if the aircraft was designed in Europe (which it was) to the European spec, then it was designed to a MTOW of 450 kg. Just because the Australian class
allows 600kg - it doesn't mean that the aircraft was designed to that higher limit!
Had the manufacturer designed the aircraft to a higher limit, then the Australian regs would
allow that limit - as that isn't the case, it indicates that 450 kg is the max the manufacturer allowed for.