Police, military in anti-terrorism drill

Emergency services and defence are conducting a major counter-terrorism exercise at Sydney's Holdsworthy Barracks.

A major national counter-terrorism exercise is under way in Sydney. (AAP)

A major national counter-terrorism exercise involving police, military and emergency services is under way in Sydney.

The exercise, dubbed Hermes Castle, is being conducted at Sydney's Holsworthy Army Barracks to test commonwealth and state responses to a terror attack.