"He had knives in both hands and, upon seeing me with the narwhal tusk, pointed at his midriff," Darryn Frost told Britain's Press Association.
Police confirmed the incident as terror-related, and said the suspect was wearing a hoax explosive device.
Decades of terrorism in Africa show that heavy-handedness is not the solution to a rising militant threat.
Butt, reportedly the ringleader of the London Bridge attack, was a member of a U.K.-based extremist group led by extremist cleric Anjem Choudary.
Behind the rapid response is years of training and lessons learnt from attacks in other major cities.
Revellers shouted and threw barstools at the three attackers, who killed seven and injured 49.
The attackers ploughed a van into pedestrians on London Bridge before running into Borough Market with knives and stabbing people at random.
One person was hit with a stray bullet and is being treated at hospital along with more than 40 other people injured in a knife and van attack in London Bridge and Borough Market.