The state's roads authority concedes that air quality in Sydney's M5 East motorway tunnel is lower than it would like due to “design issues” but says new tunnels being built alongside it as part of WestConnex will be superior.

Appearing before a state inquiry into the $16.8 billion WestConnex toll road, Roads and Maritime Services chief executive Ken Kanofski said the M5 East remained compliant with standards despite the fact its air quality would not be as good as the new road tunnels.

Roads officials say 'design issues' are responsible for lower air quality in the M5 East tunnels. Credit:Jon Reid

“There are a range of design issues with the M5 East which leads to it having a lower [air] quality than we would like. It is still compliant,” he said.

He did not elaborate on the design issues with the M5 East tunnel, which was opened in late 2001. The so-called New M5 is scheduled to open to vehicles in early 2020.