The sale of Caltex petrol stations to Z Energy could lead to decreased competition and higher prices, according to the Automobile Association (AA).
The Commerce Commission has approved the $785 million deal, although Z Energy must sell 19 petrol stations and a truck-stop to make sure it does not dominate the fuel market.
AA's petrol watch spokesperson Mark Stockdale said the deal gave Z about half of the national retail petrol market.
Mr Stockdale said Z was often first to increase petrol prices, and that behaviour may increase now the company had a greater market share.
However, he said the forced sale of the 19 petrol stations would address some of the AA's concerns.