This will depend on a variety of factors:
Labour charge - this is a fixed rate, exclusive of GST, usually between $90 and $120 per hour. There tends to be a minimum first-hour charge.
Parts and materials - if the plumber has the parts in their van, or if they need to pick up some extra items from a plumbing store, you will be charged for what's required. There may be a mark-up as well.
Specialist equipment costs - if your job requires non-regulation tools such as cameras, concrete cutters, wet back vacuums and similar, you will most likely pay an hourly charge on those. Make sure to discuss the pricing with your professional beforehand.
Travel cost (this may be included in the labour charge) - if the plumber charges travel costs, this is usually charged from the time the plumber leaves for your home, to when they arrive back at their base after the job. This will also include the time it takes to visit a plumbing store to get any parts. Just clarify with your professional if travel time is included in their hourly rate.
GST - the total of 1, 2, 3 and 4 above will be added, and GST (15%) charged on the total.