Police were responding to reports of an alleged domestic violence incident at a Homebush unit around 8.30am yesterday when they encountered the 52-year-old man.
During his arrest inside the unit police used pepper spray against the man.
A short time later he suffered what police say was a medical epsiode.
A crime scene is in place and the actions of police are being investigated.
The investigation will be subject of an independent review by the Professional Standards Command and oversight by the Law Enforcement Conduct Commission.