The Police charge you for trafficking generally for selling drugs to make profit. Unfortunately it can charged in situations where you buy drugs in a group for your own use and one person is collecting all the money.
There are number of other ways you can be found guilty of trafficking drugs. Obviously these charges are treated very seriously by the Court. The maximum for this charge under section 71AC of Drugs, Poisons and Controlled Substances Act 1981 is prison for 15 years.
Often this charge for possession and trafficking of drugs are heard at the same time. It became an issue for your lawyer to discus with prosecutor if you are pleading guilty to these charges.
The police sometimes misunderstand what actually happened and you were not trafficking drugs. In that case you would have a contested hearing before either a magistrate or a judge and jury.