Plans for the tunnel, which would run from somewhere near the intersection of Anzac Bridge and Victoria Road at Rozelle to the M2 motorway around Lane Cove, will be presented as an opportunity for motorists to bypass the crowded inner city.

But the proposal will be unveiled in only an early form, with little detail about the impact on multi-million dollar suburbs such as Balmain, Birchgrove, Greenwich or Longueville, nor information on where exhaust stacks in these areas would sit.

The plans are also emerging in response to traffic issues created by the WestConnex motorway, which will make it easier for motorists to drive between Sydney's western suburbs and denser inner areas.

If it is re-elected in March, the Baird government promised to explore two extensions to WestConnex using money raised by privatising electricity assets: a southern connection toward the Sutherland Shire on the F6 corridor, and a northern connection emerging near Anzac Bridge.

Speaking to Fairfax Media this week, the chairman of Infrastructure NSW, Graham Bradley, confirmed agencies were examining a plan to connect WestConnex with a new harbour road crossing.

Mr Bradley said there was a plan "which is under consideration, that's all I can say, for an inner link, which is another harbour tunnel to go from Westconnex to the M2".

"There are elements of that project that could significantly relieve traffic issues around the bays area, in my view," he said.

Mr Bradley's comments about a new harbour crossing came on the sidelines of an international summit organised to discuss the future of 80 hectares of public land between White Bay and the Sydney Fish Market, known as the Bays Precinct.

The proposed northern extension to WestConnex would emerge near this precinct, which has been slated for Sydney's largest urban renewal program since the Olympics.

But a "building cities" panel, facilitated by Mr Bradley and featuring Transport for NSW secretary Dave Stewart alongside some of summit's international experts, declined to discuss WestConnex. The overseas panelists would not have had "a chance to understand the WestConnex project in all its complexity," Mr Bradley told the room.

Speaking to Fairfax Media afterwards, Mr Bradley offered his support for the motorway and also flagged the second harbour tunnel project –a proposal which has not yet been revealed to the public.

"It's at early stages of discussion and I would say watch this space," he said.

"It's a 10-year plus project away but it could well be an adjunct to WestConnex that will have an impact on how it might affect the interconnection here with the city."

Mr Bradley said Infrastructure NSW was "very keen" to see the plans progress.

"The beauty of this inner link, which is seriously being looked at by Roads and Maritime Services ... is that it means people don't have to go on the Eastern Distributor, which is already … reaching the limits of its capacity," he said.

"So take another 10 or 15 or 20 years, there will need to be another crossing of the harbour ... and a linkage back to WestConnex."

Plans for the extra harbour tunnel have been under development for months, and are expected to be included in the next State Infrastructure Strategy, due for imminent release.

The first State Infrastructure Strategy in 2012, developed by the Coalition government's advisor Infrastructure NSW, included the so-called WestConnex motorway, which is to run largely under Sydney's inner west.

The WestConnex motorway will make it easier for motorists to drive toward the city, but will not add capacity to the over-stretched Western and Eastern Distributors. The second harbour road tunnel is being developed in this context.

An updated Infrastructure Strategy could be released as soon as next week. Asked to comment on a possible third harbour road crossing, a spokeswoman for Roads Minister Duncan Gay said: "We won't be commenting on the SIS until it's released."

Sources have told Fairfax Media the prospect of another crossing west of the Harbour Bridge and to the east of the Gladesville Bridge will be included in the next strategy. One source said there were concerns about the repeated expansion of the motorway projects through Sydney. "Where does WestConnex end?," one source said.