A man is in hospital in a critical condition after accidentally being shot in the head with a nail gun during construction work in Sydney's west this morning.

The man, in his 30s, was working on a site at Sarah Andrews Close, Erskine Park about 9am when the accident occurred, an ambulance spokeswoman said.

"He had a nail-gun nail accidentally embedded in his head," she said.

It's understood the injured man was working with another man putting pallets together when the accident occurred.

The injured man was conscious as he was taken to Nepean Hospital, the ambulance spokeswoman said.

He was critical but stable, a hospital spokeswoman said