There is no general law that absolutely prohibits young people drinking alcohol. It is illegal in some circumstances, but not in others.
It is illegal for a person under 18 to possess or drink alcohol in a public place, if they are not under the supervision of a responsible adult. The young person can be fined and the alcohol confiscated.
It is also illegal for a person under 18 years to drink alcohol on licensed premises, such as pubs, clubs and licensed restaurants. But it is not illegal for a person under 18 years to consume alcohol in a private place—for example, at home or at a private barbecue or party. A person under 18 can also legally drink in a public place if they are with a responsible adult (and provided they are not in a declared alcohol free zone).
Someone under 18 years can legally drink alcohol in an unlicensed (BYO) restaurant, but only if they are in the company of a parent or guardian who has approved their drinking.
It is an offence for anyone except the parent or guardian of the young person to sell or supply alcohol to a person under 18 years old, or to obtain alcohol on behalf of someone under 18. Similarly, it is an offence for the licensee of licenced premises to allow alcohol to be supplied to a person under 18.
It is an offence for a young person to obtain or attempt to obtain alcohol from licensed premises, including a bottle shop. It is also an offence for a young person to use false evidence of age to obtain alcohol.
The exception allowing a parent or guardian to supply alcohol to a young person does not apply to licensed premises. In other words, parents and guardians can legally supply their under 18 year-old children with alcohol, but not in pubs, clubs and licensed restaurants.
Source: A quick guide to drugs and alcohol, 2nd edition