The Labour Party is advising the new Energy Minister, Judith Collins, to keep a close eye on monopolistic behaviour by petrol companies.
Commerce Commission approved Z Energy's purchase of Caltex this year, giving it 50 percent of market share.
This week, Caltex Australia, which is not connected to Caltex New Zealand, announced it would buy Gull.
Labour energy spokesperson Stuart Nash said he was concerned about the consolidation of the companies and the possibility of petrol prices being heavily influenced.
"If I was the minister of energy, I would be sending a very public signal to Z, and to Caltex Australia, and all the players in this game that I am going to be keeping a very close eye on you.
"And if I do see anything, if there's any hint of anticompetitive behaviour, then watch out."
Mr Nash said he was confident Ms Collins would keep a close watch.