clearnet link - http://www.smh.com.au/national/customs-pulls-back-the-curtain-on-the-weird-and-wacky-ways-drugs-are-smuggled-in-20150711-gi9mdi.html
so there are 1 million pieces of mail coming in to the country per week, 588,000 air cargo shipments and 70 seizures per day
70x 7= 490 seizures per week 1,588,000 pieces of cargo/ mail per week chance of interception= 0.0003085 = 0.031%
However- this assumes that ALL packages contain illegal imports- you'd have to include estimates of drug imports to make these figures even remotely accurate, and I can't be fucked. Remember this does include people who get caught at airports, but does not include passenger numbers, so the actual figure is even smaller.
Also interesting is the fact that they seem to be looking increasingly at domestic mail-
'A new joint operation, code-named Icarus, recently intercepted 78 drug shipments moving through Victoria's mail system in just 77 days.'
Meth epidemic in Aus