Teen raped man in Sydney carpark: cops

A teenage boy who allegedly raped an adult man in an underground carpark in Sydney's west will face a children's court.

The 16-year-old approached the 53-year-old man, who was unknown to him, in an underground carpark in Mt Druitt at 1am on Sunday, NSW Police said.

The teenager allegedly pulled out a large knife and threatened the man, who tried to flee.

But the boy chased him down and sexually assaulted him, police said.

The man called police after the ordeal and was taken to hospital.

The teenager was located in nearby Whalan a short time later - the knife was allegedly in his possession.

He was charged with three counts of aggravated sexual assault and threatening bodily harm with a weapon.

He was denied bail and is expected before a children's court on Monday.