Some years ago a union where I worked issued legal advise that no person entering into an employment contract agrees to forgo basic civil liberties.At the time this advise was in regards to searches and detention. I imagine this would extend to a request for a sample. A bit hard to refuse perhaps when applying for a job but once employed who knows.
The advise also pointed out that company security persons have no power to search or detain and that even the police dont without warrant but do as a consequence of arrest. Customs, however, can do just about anything they want. In other countries you are again subject to their laws. Company management may wish to do this type of big brother stuff but do they have the legal authority in Australia?