They do not, unless it reeks of weed or something like that.
Aus Post also do not do any inspections of domestic mail/packages.
They need reasonable suspicion to open something, which they can only get from the cops or customs, and that is generally because they are watching rhe seller or buyer very closely.
this made me wonder, does the USPS do any inspection of random packages? I wouldn't imagine so. Unless it's like randomly they sometimes have a drug sniffing dog or something.