A man is dead and another in custody after a crash that police have labelled "a tragedy beyond belief".
An internal investigation has been launched over officers' involvement in the lead-up to the collision that killed a 49-year-old man on Tuesday morning.
Police were following a 43-year-old male they say was driving suspiciously at 4am near the Great Western Highway at St Marys, in Sydney west.
They lost sight of the man's Holden sedan before locating it again after it had collected a Toyota hatchback with the 49-year-old inside.
The man in the Toyota was given first aid but subsequently died.
Police found the driver of the other vehicle in the back of someone's house after he allegedly fled the scene.
He was taken to Nepean Hospital and police expect to charge him once he has undergone drug and alcohol testing.
Assistant Commissioner Gavin Wood said the death was "a tragedy beyond belief".
"You have an innocent 49-year-old male going to work - he doesn't get to work, he doesn't go home, his family don't get to see him," he told reporters.
"So you talk about tragedies ... for that poor man and his family and friends, their life has changed from this incident."
Police were never formally engaged in a pursuit, but they were following the sedan after deeming the driver was acting suspiciously.
But Mr Wood was light on detail when pressed on how the driver was acting suspiciously to trigger officers' interest.
The driver was disqualified from driving and was "well known" to police, he said.
The crash and the investigation into it will now be subject to an independent review after a critical incident was declared.
The process follows incidents when a person is killed or seriously injured in circumstances involving police.
"W've got two young police out doing their work, looking after the community, they form that suspicion and that resulted in them following the vehicle," Mr Wood said.
"The circumstances around the threshold of the suspicion will form part of that critical investigation."
