You remain responsible for collecting and remitting GST on goods you sell that you ship from within Australia if you meet the GST registration threshold. Several marketplaces, including Amazon.com.au do not collect GST if you're an Australian company shipping to Australian customers. We highly recommend reviewing your sales data to fully understand what your seller marketplace is doing on your behalf with regard to GST.
If your marketplace is not collecting GST and you are required to submit GST, be sure to inquire about a GST Transform. A GST transform from Greenback will transform your sales data to itemize out the required GST prior to exporting your sales to an accounting program. This will allow you to accurately report on the GST liabilities within your accounting system.