Weeks ahead of two by-elections in the northern suburbs, Premier Gladys Berejiklian revealed on Thursday the preferred route for the "Beaches Link" project and that 71 property owners would be affected by preliminary work.

Those owners - the majority of whom are in Artarmon and Cammeray - were to be notified on Thursday, although the government said it was too early to confirm exact property acquisition details. Other owners affected are in Seaforth and Balgowlah.

"We are absolutely committed to building this piece of infrastructure," said Ms Berejiklian, who had signalled the announcement weeks ago.

Ms Berejiklian, who conceded people had a right to be "slightly cynical", declined to say how much the project would cost or when exactly it would be opened to motorists.

The details about final costings, funding, route analysis and the timeframe for construction would be released by mid next year, following up to nine months of geotechnical investigations.

But Labor's transport spokeswoman, Jodi McKay, accused Ms Berejiklian of electioneering and said people were justified to be sceptical given the lack of detail about the likely multi-billion-dollar cost and timing of the project.

"The project hasn't moved forward at all. You can see [the announcement] is timed for the byelections," she said. "It will be just another toll road."

While not objecting to a new link to the northern beaches, Ms McKay said western Sydney was Labor's priority for roads and public transport because "that's where there growth is and where you see significant deficiencies around transport".

The Beaches Link project also raises questions about the government's eagerness to push ahead with the proposed F6 motorway in southern Sydney.

However, Ms Berejiklian said "we are committed to all of our major projects".

Under the government's plans, the northern beaches tunnel will connect to a planned "Western Harbour Tunnel" which will run underground to the WestConnex motorway at Rozelle.

Ms Berejiklian said the Western Harbour Tunnel was "absolutely integral" to the Beaches Link, and were likely to be undertaken as part of a single project.

The government will also begin "market soundings" with the private sector to decide on the best way to fund and deliver the project. It anticipates construction to take five years.

Mosman mayor Peter Abelson said the council wanted the tunnel link but he questioned "to what extent it is a thought bubble three weeks before a by-election".

The government said motorists were almost certainly have to pay tolls to use both the Beaches Link and the Western Harbour Tunnel once opened. The work over the coming year will consider what motorists are likely to have to pay in tolls.

"I am expecting there to be a toll - I'll be honest about that - and that will be part of the funding solution," Ms Berejiklian said.

"If we had this conversation 30 years ago, I think the community would be upset about a toll. Thirty years on, I think people expect when there is a new piece of road infrastructure that they will have to pay for it."

She also foreshadowed that more property owned were likely to be impacted as work on the project progressed.

The Beaches Link will bypass up to 19 sets of traffic lights and reduce travel times between Brookvale and the CBD by up to 40 minutes.

The announcement comes as campaigning ramps up ahead of the byelections in the North Shore and Manly caused by the retirement from politics of Jillian Skinner and Mike Baird.

Successive governments have for decades been promising solutions to road congestion between Sydney's northern beaches and the central business district via the Spit Bridge.

The so-called Beaches Link will cross under Middle Harbour and connect to the Burnt Bridge Creek Deviation at Balgowlah. It will also involve the upgrade of the Wakehurst Parkway to two lanes each way between Seaforth and Frenchs Forest.

The government has set aside $77 million for geotechnical work along the route, which will start just before Easter.

The work will take up to nine months in suburbs including Balgowlah, Seaforth, North Sydney, Neutral Bay and Cammeray, and involve 235 borehole sites.

The NRMA said the $77 million in funding for preliminary work would start a long-awaited process to bring relief to motorists and bus commuters.

"The endless delays experienced daily by residents along the Northern Beaches is simply unacceptable. We need this tunnel to make it easier for ... getting local residents home to their families on time each night," NRMA local director Tim Trumper said.

The planned Beaches Link.