Actually the threshold is if the duty + GST on the imported goods is less than $100, they don't bother charging. Practically, that means that for headphones the threshold for the product is a bit over $1000... but YMMV.
Generally, all goods imported into Australia are subject to Customs duty and GST and are assessed for community protection risks. However, all goods (except for tobacco and alcoholic beverages) may be imported duty and tax free if the value of the goods is A$1000 or less.
I doubt 'a bit over' is acceptable to customs as you must declare its real value.
Please provide evidence of the "$100 threshold"