Rail link closed after worker injury

Work has been suspended on a section of Sydney's North West Rail tunnel site where a worker was seriously injured by a falling boulder.

The 39-year-old man was welding when a 40kg boulder tumbled about 25 metres down the concrete wall and struck him from behind on Thursday.

After being stabilised on a stretcher, he was lifted to the surface by a crane before being taken to Westmead Hospital by road ambulance.

The CareFlight rapid response trauma team were also called to the site just after 6pm on Thursday.

The construction worker, from Dee Why in Sydney's northern beaches, was said to be in a serious but stable condition on Friday.

Work has been halted in the area where the accident happened while an investigation is carried out, a spokeswoman for Transport for NSW said on Friday.

"Safety is the number one priority on all North West Rail Link construction sites and TJHD (the contractors) will be conducting an investigation into the incident," she added.

A WorkCover spokeswoman said an inspector would be attending the site on Friday to investigate the incident.

The accident took place just two days after Premier Mike Baird visited the site to see the first tunnel-boring machine drill through the wall to reach the other side.

The rail link is Australia's largest public transport infrastructure project and will see the building of 15km of twin tunnels between Bella Vista and Epping.

It is due to open in 2019.