As of changes made to Australian tax law in August 2018, Facebook Ads purchases are subject to GST of 10%. As stated by Facebook, “This applies to advertisers whose 'Sold to' country on their business or personal address is set to Australia and who haven't added their Australian business number (ABN) to their Facebook ad account. If you're running ads for business purposes, Facebook is required to confirm your GST registration through your ABN.

Facebook has advised on their website that if you add your Australian Business Number (ABN) to your Facebook Ads profile, you will not be charged GST on your advertising invoice. For clarification, this is the statement provided by Facebook:

“In the Ad accounts settings of Ads Manager, you can add your ABN so it shows up on your ads receipts. If you add your ABN to your account, Facebook doesn't add GST to your purchase of Facebook ads.”

Check out Facebook’s help article for more information: https://www.facebook.com/business/help/312056985996758?id=287849115238870