A man who was restrained and handcuffed by police after allegedly assaulting paramedics in Sydney's southwest has died a short time later.

The man allegedly assaulted the NSW Ambulance officers at a Canley Vale service station yesterday before police were called about 3pm.

Concerns were raised about the man's "behaviour", police say.

He was handcuffed before being taken to Liverpool Hospital by paramedics, but died there at 3.45pm.

A critical incident investigation team will probe all aspects of the incident.

Police couldn't confirm whether or not he remained in handcuffs on his way to hospital.

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