Personally I think there are a number of factors that converge to create challenges, for one nearly all international mail entering Australia comes in via the Sydney mail center. Customs officers also work in teams for several months at a time so they get their eye in for spotting stealth and packaging techniques so they can profile some methods fairly easily. Customs services have also had their funding cut so I wouldn't be supervised if they were trying to justify their existence this year so they can say "hey look at all these drugs coming in, if you cut our funding Australia will be flooded with dangerous drugs, think of the children!" I'm sure a lot of drugs also just get caught up in the bio security dragnet but like I said I suspect its more a combination of factors making AU a country you need to up your stealth methods for.