If you sell to an Australian, you have to add 10% GST and send it to our ATO. For that you need a tax registration number,
You only need a gst registration for 2 things. 1) If you cross the gross sales threshold, which is above 80k last time I looked, and this makes it mandatory to be registered, and 2) if you want to claim back the gst you pay out making those sales.
If you're not grossing 80k a year or more, its not worth the hassle of being registered for gst. But it means you pay it out, without being able to claim it back.
What Facebook are doing, is totally illegal in Australia. Adding 10% if you don't have an ABN means FB have zero understanding of how GST works in Australia. The ABN has nothing at all to do with GST. You need an ABN to be registered for GST, but the GST registration is a different thing entirely. They should be charging the 10% GST to everyone, by including it in the price charged. Those with GST registration get to claim it back, those without dont. Facebook itself doesn't even need to know if you are registered or not, but do need to issue a tax invoice which states how much GST was charged. (If it doesn't have the words 'tax invoice' on it, its illegal.) They should just be collecting it from everyone.
The main thing is, if you're gross sales are less than 80k, you dont need to be registered for gst.
As far as I know, if you're not Australian, you cant even register for GST. Facebook for example has to, because Facebook has an Australian registered company it operates out of in Australia. So does Amazon. Both do more than 80k gross a year through those companies, so both have to be registered for GST here.
I refuse to do FB ads now, until they wise up and start handling GST properly. I dont have an ABN, dont want an ABN, and dont need one. I therefore cant quote one to cater for corporate idiots who think GST is the same as VAT. It's not.