well BK i think you answered it yourself
MAF have done disease risk assessments on introducing birds and have decided it is not worth the risk. for example if New Castle disease were to get in it would cost the poutry industry multi-millions (it is in Ozzy BTW). other potental diseases include parrot pox virus and avian influensa +
the unknowns. its economic as well as environmental reasons
quarintine for birds would require unbias staff to man it, negative pressure systems to control airflow, so not airborne disease could escape, etc. so quarintine would be very expensive.
the birds would remain expensive so smugglers would still be motivated, i think smuggling may even increase as it would be easier to explan where your new birds came from (i breed them from legally imported birds etc).
As far as returning Macaws back to brazil, i think this is unlikely to happen from NZ for your example. as the Green Wing Macaws here are probably highly inbreed and many of the have been hybridized Scarlets (i do conceed that this is the result of not being able to import new blood lines to NZ).
As far as these expensive birds not being released it is possible that some misguided animal rights activist releases your/someone elses birds because they think they are doing the right thing by freeing the birds.
S-C toos, galahs, rosellas and rainbows lories have all established in NZ.(i think the rainbows have been culled) the most likey source is caged/aviary birds. wild S-C cockatoos and eastern rosellas are known to carry PBFD (these are facts BK).
i agree that there is bias in the process. i guess it is to hard to stop ppl owning cats as too many ppl have them.