Cetnik, once you have finished with this thread, get another username because they aviation community is quite small and you will be identified quite quickly if you continue to post on Pprune under that name while training.
The ignore the likes of NAS1801 comments. He doesn't know what you and I know. For starters, there would not be one teenager in Australia who hasn't puffed on Marijuana at least once, it's far easier for teenagers to get than cigarettes and alcohol.
The drugs issue is one of those "I wish you hadn't told me that!" areas.
If you tell the DAME, then they are required to take action. Once that action is taken and its on your record, even if the DAME then issues you a medical certificate then it appears unlikely any employer will hire you, ever. It's a catch 22 situation.
To put it another way, there is no point even attempting to start an aviation career if evidence (from you or anyone else) exists of prior drug use, even if you are completely "clean" for years.
I often have to go out of my way counselling various young people that what they do before the age of 18 DOES count as far as police and medical records go. Most of them think the slate is wiped clean at age 18 - it isn't.