The former head of the Australian Competition and Consumer Commission has warned that plans by the commission to replace a "misuse of market power" test with an "effects" test could leave consumers worse off.

Key points: Harper competition review recommended change from "misuse of market power" to "effects" test

Harper competition review recommended change from "misuse of market power" to "effects" test Change would mean the ACCC does not have to show large firm intended to reduce competition

Change would mean the ACCC does not have to show large firm intended to reduce competition Former ACCC boss Graeme Samuel said that could prevent big retailers offering cheap staples

Former ACCC chairman Graeme Samuel said consumers could miss out on cheap grocery staples such as bread and milk if the ACCC is allowed to sue big businesses for taking any action that could reduce competition.

The effects test was a recommendation of the Harper Review as a way to protect small businesses from large corporations who abuse their market power.

The test relates to Section 46 of the Competition and Consumer Act, where the threshold definition of "substantial market power" is currently very high.

The ACCC's main gripe about the current law is that it is very hard to prove that a large firm intended to reduce competition through its actions, whereas an effects test would mean the regulator only needed to show that the result of a firm's actions was reduced competition.

Mr Samuel said the proposed ACCC plan to abolish this would give smaller businesses an unfair advantage, and it would cost the everyday consumer.

"The High Court has said on many occasions that business, whether it be big or small, when it engages in vigorous competitive behaviour, might in fact harm certain competitors, it could even reduce the level of competition in a market - in most cases it will produce a more competitive environment," Mr Samuel told ABC News Radio.

"But we shouldn't have big businesses being penalised for engaging in pro-competitive behaviour.

"In relation to big business, the law has always said that big business can't misuse its market power or take advantage of its market power to cause damage to competitors."

The current head of the ACCC Rod Sims disagrees, and told the ABC in September the problem is the way courts over the past decade have interpreted the words "take advantage".

"They've interpreted it to mean that a firm with substantial market power can take whatever steps it likes to exclude its competitors, say by buying up all the land or all the inputs to production," he argued.

"It can do that, provided that the steps it takes are ones that a firm without substantial market power could also have undertaken."

Mr Samuel was chairman of the ACCC from 2003 to 2011, before Mr Sims took over.