A decade on from the shocking slaying of an unarmed police officer, a state's top terrorism cop admits tracking down radicalised people is harder than ever.

NSW officers on Thursday recalled the life of police accountant Curtis Cheng, who was gunned down by a radicalised teenager on October 2, 2015.

The assailant - 15-year-old Farhad Mohammad - was subsequently killed in an ensuing gunfight with guards outside the NSW Police Force headquarters.

Deputy Commissioner David Hudson, the state's lead anti-terrorism officer, used the 10-year anniversary to reflect on the changing extremist threat in the past decade.

While police were focused on ISIS and al-Qaeda in 2015, Mr Hudson said the "threat mix" had evolved to include many different ideologies.

That made monitoring them far more comp

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