The buyer is responsible for paying customs duties, if any.
In general, packages with a value of less than US$200 will not require any duty charges.
If multiple packages from the same location are received in the same day, the value will be summed and duties are owed if the value is over US$200.
Imports with a FOB value, i.e. product value excluding shipping and insurance cost, up to AU$ 1000 are exempt from duty, GST and Import Processing Charge so you usually don't have to worry about that.
Please consult your customs office to inform you about possible customs and duty regulations.