In order to list on Amazon the retailer must supply the product barcode (GTIN / UPC / EAN) information.
Amazon does not create product barcodes for individual products. Amazon can generate a GTIN (barcode) as part of the FBA (fulfilment by Amazon) programme to track parcels in their warehouse but this is not a product specific barcode.
A retailer can purchase barcodes from a number of companies in Australia or find the product barcodes by utilising a UPC Lookup tool.
You can check GTIN existence here https://gepir.gs1.org/index.php/search-by-gtin