Customs in Australia are a bit of a cowboy outfit and take liberties with the laws and their interpretations. They go on seizure benders and regardless of whether the rules are followed may seize and destroy without any appeal process. They can decide randomly that any packing material is soil or that the absence of a phyto certificate is grounds for destruction.All well and good for an iron fist with biosecurity if there is consistency.
While mail really hasn't been shown as a source of pests the ports are a different tale. Fire ants, yellow crazy ants, papaya fruit flies, asian honey bees and a thousand other pests stream in through Australian ports. State governments actually encourage the use of some of the world's worst weeds like buffel grass and leucaena. There just is no coherent, sober or coordinated approach to biosecurity.