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