The Grocery Commissioner says the returns our big supermarkets get on assets is about double what overseas equivalents make.

The Commerce Commission's first annual report into the grocery market shows competition hasn't meaningfully improved.

It flags increasing margins and high profitability.

Pierre van Heerden says overseas grocers make less because they have more competition.

"The return on assets is about 11 percent vs 5.5-6 percent overseas."

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