The competition watchdog has flagged possible concerns over Transurban’s interest in taking a majority stake of Sydney’s WestConnex motorway, saying the toll road operator’s dominance in NSW could stymie competition in the sector.

Australian Competition and Consumer Commission chairman Rod Sims says Transurban’s position as the operator of seven of NSW’s nine existing toll roads means it has highly detailed traffic data that is useful when compiling bids for other projects.

Mr Sims cited the fact that Transurban has been granted five toll road concessions following unsolicited proposals to state governments, and is the only company to have succeeded with an unsolicited bid in the past 30 years.

“WestConnex is a very significant toll road asset, and as such represents an opportunity to establish a strong rival toll road operator,” Mr Sims said on Thursday.

“We are concerned that the proposed acquisition may cement Transurban’s advantages when competing for future toll roads.”

The NSW government is aiming to sell 51 per cent of the company behind WestConnex, a 33-kilometre route of interconnected motorways and upgrades that could enable motorists to travel about 50 kilometres from Sydney’s west to the edge of the CBD without traffic lights.

Mr Sims said two competing toll road operators in the state would likely compete for future projects and vie for government approval for unsolicited proposals.

They would also offer drivers a choice in instances where WestConnex competes with existing Transurban roads.

The ACCC has set a May 31 deadline for responses to the preliminary views published on Thursday.

Transurban chief executive Scott Charlton said the company and its consortium partners would continue to assist the ACCC with its review.

“The release of a statement of issues by the ACCC is a recognised step in the ACCC’s informal merger review process and, as we have done on previous acquisitions, we will continue to work with the ACCC to enable them to complete their review,” Mr Charlton said.

At 1026 AEST, Transurban shares were 25.5 cents, or 2.2 per cent, lower at $11.295.

They had already fallen 3.1 per cent over Tuesday and Wednesday.