A man is dead and another in custody after a crash that police have labelled ‘a tragedy beyond belief’.

Police were following a 43-year-old man 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 collided with a Toyota hatchback.

The 49-year-old man in the Toyota was given first aid but subsequently died.

The Holden driver allegedly fled the scene but was found behind a house.

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.

‘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.

The driver was disqualified from driving and was ‘well known’ to police, Asst Comm Wood said.

The crash and the investigation into the police role in it will now be subject to an independent review after a critical incident was declared.

‘We’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.’