Woolworths and Coles would be prevented from opening more stores and banned from owning more agricultural land under a tough new competition policy to be unveiled by the Greens on Thursday.

The Greens would also boost the power of the Australian Competition and Consumer Commission and lift its legal budget by 50 per cent.

Tough new policy: the Greens would prevent Woolworths and Coles from opening more stores. Credit:Jim Rice

To be revealed at a symposium on supermarket power in Melbourne, the policy would place a temporary ban on expansion by the two supermarkets giants while the ACCC carried out a desktop review of the effect of its previous decisions on the dominance of the chains. The review might last up to a year.

Greens competition and small business spokesman Peter Whish-Wilson will say the chains manage or own a large part of the supply chain, including retail premises, warehousing and delivery and the supply of branded products. They also own wine processing and brewing companies.