A Sydney construction worker has been hit by a falling boulder while working near the entrance to the North West Rail Link tunnel.

Careflight said the 39-year-old was hit by the 40-kilogram boulder while welding at Baulkham Hills in Sydney's west.

He was treated by paramedics at the scene for cuts and lacerations before being taken to Westmead Hospital on Thursday evening.

"A rock had become dislodged on the surface and tumbled about 25 metres down a concrete embankment and struck him from behind," said Mark Lever from Careflight.

"He suffered a number of injuries and was treated at the scene for lacerations and cuts and then carefully raised to the surface."

The injured man from Dee Why is in a serious but stable condition.

On Wednesday Transport Minister Gladys Berejiklian said the $8.3 billion project was $300 million under budget and she indicated there would be further savings to come.

The rail link is expected to open in 2019.