RSPCA says buy from a responsible breeder and sets out a list of what that means to them.
What is a responsible companion animal breeder? - RSPCA Australia knowledgebase
And this is their take on "registered" breeder. I guess "registered" is about as meaningful as "natural" or "fresh" on food labels.
What is a 'registered breeder'? - RSPCA Australia knowledgebase
If you read between the lines, RSPCA thinks that there needs to be another level of recognition for "responsible" breeders, and the current ambiguity of "registered" breeder is not it. Ie you can be registered with what ever system and still not be operating with the best interests of the animal at heart.