Global companies with a presence in Australia prevent local customers taking advantage of cheaper prices offered on their overseas websites, consumer group Choice says.

In its submission to the Productivity Commission's retail inquiry, Choice says Australian retailers must explain why they charge huge mark-ups on products that cost half as much from overseas internet companies.

A strong Australian dollar should enable local retailers to pass on savings to their customers, according to Choice.

It has found that the same pair of Nike running shoes costs $240 at a major Australian sports retailer while consumers can buy the same shoes for $134 from an online store based in the US.

The Choice report follows one by the Australia Institute earlier this week, which compared the perception by consumers of retailer markups and the reality. The difference between wholesale and retail prices was narrower for industries where online retailing is more active.