I have read articles by vets that say Quarantine isnt 100% effective , as a true carrier bird is never cured even after medication , a carrier bird can supress illness for 12 months or more before showing the signs of illness and in this time spreads the disease .
Birds have a built in factor from the wild that stops them from showing weakness , this is so predators dont get them straight away, you will never find this in quarantine.
Even though I dont breed anymore , I think putting birds under breeding stress is the easiest way to see if they are carrying anything.
With todays birds and all the problems that have been bred into them , Quarantine is almost obselete.
Goodluck with yours Melbournebudgies, Looks to me like you will need to Doxy every year from now on , thats why big breeders do it. Our pets and stock are worth too much not too!!