The state government has rejected the only bid it has received from the private sector to build a major road interchange at Rozelle in Sydney's inner west, regarded as the most complex part of the $16.8 billion WestConnex toll road project.

In an about-turn, the state's roads agency, Roads and Maritime Services, will now take control of the process to select a consortium from Sydney Motorway Corporation, an entity the government set up to deliver Australia's largest toll road.

The government insists the setback in finding a builder of the underground spaghetti junction will not alter its plans for construction of the entire 33-kilometre toll road, to be completed by 2023, or the sale of a 51 per cent stake in the project next year.

Following questions from the Greens, senior Liberal MP Don Harwin blamed the lack of bidders for the interchange on "general market conditions" and the significant amount of infrastructure under way in NSW.