I just googled it as Im sure in NZ its 14yrs, but seems there is no legal age here, which is a bit of a shock to me!
Here is something I got off another site...

[QUOTE] What does the law say in Australia?

There is no actual law that states at what age children can or cannot be left alone, but the law is clear about the responsibility of parents to look after their children.

In many cultures it is usual for children to care for brothers and sisters.

While different societies have different customs, in Australia there is a legal obligation for parents to make sure that their children are properly looked after.

Parents are expected to provide food, clothing, a place to live, safety and supervision.

Parents can be charged with an offence if children are left in a dangerous situation and are not fed, clothed or provided with accommodation (in South Australia this is part of the Criminal Law Consolidation Act).

The police or Children, Youth and Family Services (part of the Department of Health in South Australia) can remove children from situations where their safety is in serious danger and there is no guardian present (Children's Protection Act, South Australia).[\QUOTE]

So you could leave him, but i think your totally right to choose not to.