An artist's impression of a bridge that will carry the rail line over Windsor Road at Rouse Hill. One issue is that contractors have discovered asbestos at a construction site for the skytrain, which is to run for four kilometres above ground between Bella Vista and Rouse Hill. And Transport for NSW has also agreed to extra payments to joint venture Salini-Impregilo to resolve a design dispute over the loading capacity of the elevated rail line. A spokesman for the department said: "The skytrain contract value has now been revised to $389.7 million." The big risk for the government is if further delays hampered other sections of the North West Rail Link project.

The 23-kilometre North West Rail Link. Credit:Adrian Nesbitt The rail line, which has a total budget of $8.3 billion, is due to open before the 2019 state election and will involve closing the existing Epping to Chatswood rail line down for more than half a year. But a spokesman said there had been no change to the scheduled finish of the skytrain, which is to be handed over to other contractors in 2017. Multiple sources said one problem with the skytrain project had been a high level of turnover of senior managers working for the government and Salini-Impreglio, a newcomer to Australian construction. And Salini-Impregilo had found it difficult to fill positions.

Sources said there was a perception among experienced engineers that working for the newcomers could put them offside with the dominant players in Australian construction - Leighton and Lend Lease. The Transport for NSW spokesman confirmed the contract was "adjusted to allow important work to be completed on time", and listed a number of changes. These included changes to the layout of the bus terminal and car park at the Rouse Hill town centre, changes to the overhead wiring structures on the skytrain viaduct, and changing the design of the skytrain viaduct. It is understood Transport for NSW and Salini-Impregilo differed over the loading gauge of a section of the skytrain. Transport for NSW was insisting on an elevated railway that could carry heavier trains while the contractor insisted it needed only build an elevated line with a lower weight limit.

But the Transport for NSW spokesman said the skytrain would be able to carry as many trains as needed. "The skytrain will easily accommodate fully-laden eight-car rapid transit trains including running them at higher frequencies as demand increases," the spokesman said. Under former Transport Minister Gladys Berejiklian, the North West Rail Link project was split into three major contracts. The first was a $1.15 billion tunnelling contract. The second was for the skytrain, while a contract to fit out the stations, track and signalling and to run the trains was signed in September. Greens MLC Mehreen Faruqi said the 15 per cent variation on the contract raised questions about the government's ability to manage contracts competently.

"If this is replicated across the North West Rail Link project, the government will be wasting hundreds of millions of dollars of public money on variations, for a project that doesn't even integrate with the double-deck Sydney Trains network," Dr Faruqi said. The cost of the proposed light rail line through central Sydney has also blown out by about $500 million, from $1.6 billion to $2.1 billion. In a statement, Transport Minister Andrew Constance said: "The North West Rail Link is Australia's largest public transport project and the NSW government is proud to be delivering it. "We made clear back in 2013 that the final skytrain price may vary due to the unique three-contract structure and that any change would be covered in the overall $8.3 billion budget, which has not changed." The Transport for NSW spokesman said asbestos remediation work was expected to finish in July.