Bunnings has slammed a push by Woolworths for the code governing supplier relationships to be expanded to cover rivals Amazon, Costco and Chemist Warehouse, saying it would do little to lower prices and imposed a one-size-fits-all approach on unrelated business operations.

Coles and Woolworths signed on to plans to make the supermarket code of conduct mandatory on Monday, as Labor backed proposals from former minister Craig Emerson to impose penalties of up to $5.2 billion for chains found to be abusing their power.