But, why unanimous consent and not a simple 2/3 majority? Also, why was such power given to the Attorney General and not the Governor or legislative branch?
the governors are a position of formality; they hold no real power whatsoever. and even if it were, you'd be in the same situation. if you really meant "premier" then i don't know; probably because it is assumed that any AG serves at the whim of his premier.
as for a 2/3 majority, im afraid i can't answer that. its been a while since i studied the constitutional framework. i suspect it was to ensure that all the states had an equal say in the decision(s). you have to realise the australian constitution was a game of placation; they had to have all states on board in order to form the nation. smaller states were worried about having their decisions countermanded by larger ones.
its really not that big a deal. australia only has 6 states, so asking for uniformity isn't too much of a problem. indeed, i wouldn't have even realised this necessity had it not related to gaming in some way, i suspect its due to trade or weights/measures provisos in the constitution. couldnt tell you exactly under which section "classification" falls though.