The OP is looking for fares ex UK to AU ( plus economy and premium economy) and for that the fare rules for classes O,T,I and A ( as exmples of lowest fares per class ) have no such restriction

Indeed there can be restrictions , if the carrier imposes them in the rules, but for the OP's issue, it seems to just be that may need to pick up the phone and make the booking if the website is not set up for such a purchase