In what will be the biggest single funding announcement of her campaign so far, the Prime Minister, Julia Gillard, will promise $2.1 billion in federal funds towards the project, with the state government to contribute $520 million.
Preliminary work on the 14-kilometre heavy rail link, which will shave 25 minutes off the trip from Chatswood to Parramatta, will begin next year and construction is due to be completed in 2017.
To ensure the state government delivers on its end of the deal, the project will be overseen by the federal government's infrastructure body, Infrastructure Australia.
Ms Gillard will be joined by the Premier, Kristina Keneally, for an announcement today in the western suburbs. It will precede Ms Gillard's one-hour town hall-style meeting at Rooty Hill RSL.
The Opposition Leader, Tony Abbott, will have his own one-hour session after Ms Gillard, having again declined her invitation for a debate.
Ms Gillard and the Transport Minister, Anthony Albanese, will announce today that the upgrading of the M5 East, and the F3-to-M2 link, will also be overseen by Infrastructure Australia.
Planning studies into options for both projects are due by the end of the year but the federal government will say today that both will need significant private sector financing.
Options being considered by the reviews include tolls and an ''availability payment'' model, in which a private company builds the extension and leases it back to the government.
The M5 East expansion may include another tunnel alongside the existing one which carries 95,000 vehicles a day.
Along with Queensland, western Sydney is a key election battleground. The Parramatta-Epping rail link will stretch between the Labor-held seats of Bennelong and Parramatta.
It will also alleviate pressure on the city-bound western line which services commuters in other key marginal seats further west such as Lindsay, Greenway and Macquarie.
The new link will consist of a tunnel from Parramatta to a new station at Rosehill-Camellia. The existing single track from Rosehill to Carlingford will be made a two-way line. Carlingford will then be connected to Epping by a new tunnel. The stations at Parramatta, Rydalmere, Dundas, Telopea and Carlingford will be upgraded.
Mr Albanese told the Herald the link would not be a cure-all for congestion problems in the west but it would be an important first step.
''You can't solve all of Sydney's transport needs in one step,'' he said.
The state government contribution will be spent first with the federal funds not scheduled to be available until 2014-15, by when the budget is due to be back in surplus.
Ms Gillard will say the project meets her pledge to try to ease the pressures caused by rapid population growth.
Today's announcement will also signify a rapprochement with the state government.
The federal Labor government under Kevin Rudd was loath to give it any significant infrastructure funding because of its reputation for mismanagement and poor service delivery.
For this reason, the NSW government will be required to work with Infrastructure Australia.
If Labor loses the state election in March, the NSW funding for the project which Ms Keneally identified in her $10-billion, 10-year transport plan, will be locked into the forward estimates and pressure would be on the Liberals to keep it there or lose the project.
The announcement will be one of several big promises being unveiled by Ms Gillard as the election nears.
Mr Abbott chose to stay away yesterday from the Coalition's alternative broadband policy launch which proposes to scrap the government's $43 billion project and replace it with a $6 billion system which would be much slower than the government's.