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.

In the interview, which aired on Tuesday night, Mr Gay could not tell Channel 7's Lee Jeloscek how many cars were expected to use the WestConnex, how many cars used the existing M4 and M5 motorways, and how many exhaust stacks would be needed for the project.

"I don't feel that hot about getting one wrong," Mr Gay said, at a press conference in which the Baird government promised an extra $300 million over 10 years for road improvements in southern Sydney.

"No one is caning me worse than I'm caning myself on that because I missed an opportunity to put a message out," he said.

"There are 100,000 cars that use the M4, 100,000 cars use the M5, 300,000 will be on WestConnex, and importantly 120,000 cars will be on Stage 3, and that is the missing link that the Labor party wants to remove."

Mr Gay said if Labor had its way, those 120,000 cars would be spewed into suburbs such as Newtown, Balmain, Leichhardt and Erskineville.

"But I'm confident they won't be because we will have a magic tunnel that will go underneath and King Street will become the nirvana that we always hoped it would be and we always thought it would be and I might even try and buy a black shirt," he said.

The forecast of 120,000 cars a day using the tunnel between St Peters appears to be a new claim about the motorway link, which is not expected to be open to traffic until 2023.

If accurate, the forecast would immediately make the tunnel one of the city's busiest roads. About 120,000 cars a day currently use the M2 Motorway in northern Sydney, about 128,000 currently use the M5 motorway, while fewer than 40,000 cars a day use the Cross City Tunnel.

The Opposition's plan is to build new M5 East and M4 East motorway tunnels, but change the route and exit locations. Labor says a tunnel linking the two motorways is not necessary, a claim contested by the motorway proponents.

The chief executive of WestConnex, Dennis Cliche, told a public forum on Monday night the link between St Peters and Rozelle was essential.

And even some opponents of the combined motorway projects support the idea of a tunnel bypass of the inner west. The Mayor of Marrickville, Mark Gardiner, told the forum at Enmore Theatre that the WestConnex would make traffic worse, but focused on the four-year wait for the construction of the third stage to start.

"If this motorway is going to be built, Stage 3 needs to be advanced," Mr Gardiner said.

"We cannot allow 100,000 vehicles a day to impact on the existing surface roads of the St Peters area," he said.