You are a sub-trade contractor if you work for a licensed trade contractor. You don't need a licence because the responsibility for the work is covered by the licensed trade contractor.
There are exceptions. Where the work is plumbing, drainage, gasfitting, termite management - chemical or fire protection, as a sub-trade contractor you need to hold an occupational licence. Where the work is for a completed building inspection, you need to hold a contractor licence.
You will also need to apply for a licence if you decide to contract directly with a builder or owner.
The downloadable Sub-trade Contractors Fact Sheet (PDF) gives examples of when a licence is/is not required.