[flyflewflaw]

Australian businesses collect a value added tax (called GST) on almost all sales (some exempt goods). The same tax is payable on sales in Australia and imports.

For big intl retailers the effect is as you describe, they collect and remit the tax, then their shipments are considered tax paid when imported otherwise they would be held at customs.

It probably wouldn't be that hard to audit just with data from the major payment processors.