On what kinds of transaction do you pay GST?

If you are an Australian organisation selling goods and services online in Australia, then the Goods and Services Tax (GST) will apply to all online transactions, except for those that are GST-exempt.

Generally, if GST applies to a transaction in the offline (physical) world then GST will also apply to a transaction for the same item online.

If, however, the purchaser of the goods or services is from outside Australia, then GST is not applicable. Nor is GST applicable if an overseas organisation sells goods or services with a value of less than A$1000 to an Australian customer. However, if a foreign organisation is doing business from within Australia, then GST (and all other regulatory and taxation) obligations apply.

Whether you are trading online, in the physical world, or both, it is important to understand your GST obligations.

The Australian Taxation Office website has complete information on its page
GST essentialslink