Under the Domestic Animals Act 1994, all dog owners must securely confine dogs to the property. This means your yard must have a closed gate, and an escape-proof fence that your dog cannot jump, get under or through.
Where the owner of a dog is under the age of 18 years, the parent or guardian of that owner will be deemed the legal owner of the dog and subject to any penalties/prosecutions.
You may face legal consequences if your dog rushes at or chases someone.
If your dog rushes at or chases someone, you could be fined, and your local council can declare your dog to be a 'menacing dog'. This means you will have to leash and muzzle it in public at all times including in any dog off leash area.
If you do not comply with these requirements, council can then declare your dog to be a 'dangerous dog'. There are very strict controls on the housing, exercise and ownership of dangerous dogs.