Critical incident declared after man dies in crash

A critical investigation has been launched after a man died in a crash.

The incident unfolded about 4am on Tuesday after two officers in a fully-marked police vehicle saw a dark-grey Holden Insignia sedan stopped on Mamre Road at St Marys.

Police had stopped near the sedan when the driver alleged accelerated towards the Great Western Highway, where police lost sight of the vehicle.

Police followed and found the sedan had hit a silver Toyota Corolla hatchback.

The driver of the sedan allegedly fled the scene on foot.

The driver of the hatchback died at the scene; he is yet to be formally identified but he is believed to be in his 40s.

A 43-year-old man, alleged to be the driver of the sedan, was arrested a short time later on Chapel Street.

He has been taken to Nepean Hospital under police guard for mandatory testing.

A crime scene has been established and a critical incident team from North Shore Police Area Command is investigating the circumstances surrounding the incident.

The investigation is also subject to an independent review.

Anyone with dashcam footage, CCTV vision or information about the incident is urged to contact police or Crime Stoppers on 1800 333 000.

The Great Western Highway is closed in both directions at St Marys, between Glossop Street and Sydney Street.

Light vehicles are diverting via Glossop Street, Brisbane Street and Sydney Street in both directions. Heavy vehicles can use Mamre Road, the M4 and the M7. Drivers should monitor www.livetraffic.com for updates.

Anyone with information that may assist investigators is urged to contact Crime Stoppers: 1800 333 000 or https://nsw.crimestoppers.com.au. Information is treated in strict confidence. The public is reminded not to report information via NSW Police social media pages.