Getting your GST settings right from the start is absolutely essential, so well done for tackling it.
The Short Version: You are responsible for GST, and you must configure your tax details within the Commerce Manager. Best practice in Australia is to list your prices as GST-inclusive.
How you configure these settings directly impacts the price, which is the most critical piece of text content on your product image listings.
You will find the tax controls within your Commerce Manager under a section typically called ‘Settings’ and then ‘Taxes’, which is where you must enter your ABN. There will be a crucial setting that asks if your prices include tax. For selling to Australian consumers, you should select this option and ensure the prices you enter for each product already have the ten per cent GST factored in. Facebook does not remit the GST to the ATO for you; it provides you with sales reports which you then use to complete your BAS. Think of Commerce Manager as your cash register, not your accountant.
Cheers,
Jeff