EVERY new passenger trip on the north-west rail link will cost the state about $80, correspondence prepared inside the NSW Treasury and released by the state opposition says.

The email, from senior Treasury official Rodney Forrest to the general manager finance at RailCorp, Peter Crimp, asked Mr Crimp to assess assumptions Treasury had made about the per passenger cost of journeys along the proposed line.

Adding up ... the new north-west rail link could be costlier than first anticipated. Credit:Wolter Peeters

''In 2021 the CityRail network is forecast to have approximately 391 million total trips,'' the email, dated July 29, says.

''If the assumptions are sound the NWRL may carry only an additional 9 million new rail passengers (excluding 19 million passengers who may divert from other lines) representing only around 2.15 per cent of total forecast rail patronage … The cost per trip may be around $80 per new passenger (or $30 for every passenger), compared with the 2010 average cost per passenger of $10.61.''

Asked about the contents of the email by the opposition transport spokeswoman, Penny Sharpe, the Transport Minister, Gladys Berejiklian, fiercely defended the government's commitment to build the line, previously estimated to cost $8 billion.

''Are you suggesting at all that we shouldn't build the north-west rail line? There are hundreds of thousands of people who live in the north west who do not have access to mass transit systems,'' Ms Berejiklian said yesterday.

The government has said it would release a cost estimate of the project, a 23-kilometre line stretching from Epping to Rouse Hill, by the end of the year.