Australian Drug Information Network
This is a government funded site that aims to offer good quality internet based drug and alcohol information. They also have information specifically for family and friends and lists of helpful resources (e.g. a national directory of treatment services)

Australian Drug Foundation
This is a non-profit organization dedicated to reducing alcohol and drug problems with information on most of these substances.

Family Drug Support (FDS) Help Line
Call 1300 368 186. Open 24 hours seven days a week.
FDS is a service for families and friends of people with drug related issues. It is a run by volunteers who provide information, referral and non-judgmental listening.
They also have a website to address the support and information needs of parents and other family members of someone with problematic alcohol or other drug use:

Highsnlows has information and a discussion forum about the link between mental health problems and cannabis use.