Sydney drivers could suffer if Transurban buys the controversial WestConnex project, given it already holds a near-monopoly on the state's toll roads, says the competition watchdog.
ACCC chairman Rod Sims on Thursday said the commission was concerned Transurban's acquisition of the multi-billion dollar toll road could stymie competition.
Transurban already controls seven of the nine existing toll roads in NSW.
"We are concerned that the proposed acquisition may cement Transurban's advantages when competing for future toll roads," Mr Sims said in a statement.
"We consider that Transurban already has significant incumbency advantages when competing for future toll road projects.
The Westconnex motorway aims to improve connectivity between Western Sydney and Sydney CBD.
Mr Sims said if the $16 billion motorway was purchased by a rival bidder, it would improve competition in NSW.
"Those two players would likely compete strongly for future toll road projects and vie for government approval for unsolicited road proposals," he said.
"An alternative owner of WestConnex might lower tolls for certain trips, or make changes to its roads or services to ease traffic congestion in attempts to attract more vehicles away from Transurban roads."