Dr Lindsay Matthews said until 2019 there had been just 11 reports of a human being killed by a ram worldwide in scientific literature.
He told RNZ he was saddened to learn of the death of an elderly couple on their west Auckland property this week. Police have named the pair as Alfred Helge Hansen, aged 82, and Gaye Carole Hansen, aged 81.
A ram was found in the paddock with them and was later shot dead. Police said postmortem results indicated injuries consistent with an animal attack.
Dr Matthews, principal scientist for Matthews Research International, said the incident was an "absolute tragedy", and a ram could hit with great force with injuries consistent with being hit by a car.
He said male farm animals could be dangerous especially during breeding season, which is now on farms.
"It is an animal's natural behaviour to compete with other males for females, and that brings a whole set of behaviours. Rams can butt other males very hard, then back up and charge each other, hitting with cross force on the front of the head."
He said the behaviour can be most risky for people dealing with hand-raised animals.
"Outside the breeding season they can be very friendly and that is appealing. In breeding season, they are different. They can be very competitive and regard the human as competition, especially if they are hand-raised."
A person familiar with the ram may have their guard down, and then be charged.
Dr Matthews said a ram could weigh up to 150kg and charge at speed.
"Unless you have your eye on them all the time, you can be hit quite suddenly.
"The injuries reported are likened to being hit by a car."
Dr Matthews said once a ram started attacking it is hard to stop them.
The deaths of the Hansens has been referred to the Coroner.