Sigh. There are a lot of naive people here.
There are many many youths who regularly consult with medical professionals about things their parents have no knowledge of whatsoever. Physical, verbal and sexual abuse, drug use, sexual activity etc. The naivity of sweeping statements like "my teenage kids would never touch drugs" is simply astounding. Just look at the tragic example above.
That aside, this is not about the legality, ethics or morals of drug use, it's about being employed as a pilot with a history of recreational drug use. Discriminating for historical substance abuse is ILLEGAL under the Disability Discrimination Act 1992 (Commonwealth).