Two of 24 concrete spans erected for the "Skytrain" section of the new rail line to Sydney's north-west suburbs may need to be pulled down after cracking.

The skytrain spans, which have an average length of almost 40 metres across the project, are made up of pre-cast concrete segments locked together with steel cords.

But Transport for NSW, which confirmed the cracking after enquiries from Fairfax Media, says there should be no delay in the overall construction of the $8.3 billion Sydney Metro NorthWest project, due to open in early 2019.

On Thursday, Premier Mike Baird and Transport Minister Andrew Constance announced the early and under budget completion of tunnelling for the rail line – a mammoth undertaking that has involved drilling 15-kilometre twin tunnels.