Australian GST doesn't put US products at any disadvantage to Australian products in our market. What taxes US jurisdictions apply to goods, whether they are from the US or anywhere else is a matter for them. If some choose not to tax goods at the point of sale, that's their decision not something that some other country has done to them.
Australian and US internal tax arrangements (and their costs of production, including labour costs) each affect the prices they can afford to charge on products they export. We could claim that US national minimum wages that are lower than ours give your producers an unfair advantage selling into Australia. (We don't.)