Liberal MLC Peter Phelps "went berserk" during a partyroom meeting and vowed to not support legislation to force small petrol retailers to sell an ethanol blend.

Opponents of the legislation say it will drive up petrol prices by as much as 8¢ a litre.

Ethanol blend protest: Liberal MLC Peter Phelps.

Mr Phelps - a self-styled libertarian - upbraided the minister with carriage of the legislation, Victor Dominello, during the meeting on Tuesday.

"He went berserk," a source said.