Commerce Commission executives want the provision of an honest belief defence removed from the bill aimed at cracking down on cartel and anti-competitive activities.
Parliament's Commerce Committee is considering the Commerce (Cartels and Other Matters) Amendment Bill.
It introduces criminal sanctions when rival firms agree not to compete, by fixing prices and restricting supply.
National MPs put it to the committee that in some cases businesses could in all good conscience be unaware they were acting illegally.
But Commerce Commission chairman Mark Berry says the provision has the potential to compromise the whole act.
Mr Berry says he is unaware of such a provision anywhere else in the world.