Teen hurt in bogged 4WD in Sydney's west

A teenager has critical injuries after being hit in the head with a piece of metal as his four-wheel-drive was being pulled from a bog while off-roading in Sydney's west.

Police say the 17-year-old P-Plater was hit in the head by a shackle or strap as other drivers tried to pull his vehicle free from a fire trail waterhole off Devlin Road at Penrith on Sunday night.

It was a freak accident, police said.

"That piece of metal hit him square in the back of the head - he's still alive thank God," police inspector David Ralph told 2GB radio on Monday.

The 17-year-old suffered head injuries and was flown by helicopter to Westmead Hospital where he remains in an induced coma.