King Street in Newtown will become a "nirvana" according to Roads Minister Duncan Gay, who says about 120,000 people a day will use a planned tunnel under the popular retail strip.

The tunnel between St Peters and Rozelle - the third stage of the WestConnex - has emerged as a major election difference between the government and opposition, with Labor opposed to the plan.

But details about the motorway have remained patchy since the tunnel was first announced in 2012. And only recently has the government started to say how many cars it expects to use different sections of the WestConnex project.

On Wednesday morning, Mr Gay responded to a television news interview in which he was unable to cite key details about WestConnex.