A concussion was defined as a disturbance of the brain after a blow or violent shaking of the head.
Symptoms include temporary unconsciousness, headache, memory loss, vomiting and nausea.
"Concussions in the first instance might not be that dangerous, but if you go back on and continue playing after one, that's really dangerous - that's second-impact syndrome, which can be life-threatening," Professor Leathem said.
The Accident Compensation Corporation and the International Rugby Board both require a player to be stood down for up to three weeks after a concussion.
During the World Cup final, French player Morgan Parra took a blow to the head from All Blacks captain Richie McCaw's knee and appeared shaky as he stood up.
He was taken from the field but re-appeared shortly afterwards and played on for five minutes until he took another knock during a tackle and went off for good.
But Canadian flanker Adam Kleeberger was helped from the field after he clashed heads with All Black Tony Woodcock and did not return.
Natasha Bauer, who did the research as part of her honours clinical psychology dissertation, watched full match coverage broadcast by Maori Television and noted 95 incidents.
Professor Leathem then reviewed them and determined there was enough evidence to say there were 13 "probable" concussions.
But the NZRU's senior scientist in injury prevention and high performance, Dr Ken Quarrie, said not all blows to the head resulted in concussion and a diagnosis usually required evaluation by a doctor.
"Therefore, while some of the injuries observed during this study will have been concussions, making a call in many of the cases will have been little more than guesswork, particularly if based solely on television footage," Dr Quarrie said.
The researchers also said broadcast commentators tended to downplay the possible effects of head knocks on players.
"Players were commended if they got back up groggy - they were seen to be heroes," Professor Leathem said.
"But that's not how a serious head injury should be viewed."
The researchers felt the way sports commentators described such incidents through jokes and colloquial expressions contributed to a casual feeling about the effects of concussion.
World Cup commentators had referred to a player wobbling "like a drunken rhino" or being "knocked for six", and had played down an injury by suggesting that "he was milking that".
"The commentary didn't really take it seriously, but they have to be entertaining, I suppose," said Professor Leathem.
"If someone badly broke their leg, they'd take that seriously, but with concussion you often can't see it."
Maori TV's general manager of programming, Haunui Royal, said the research indicated that as a broadcaster of sports programming, the channel needed to give further consideration to how the effects of concussion were portrayed by commentators.
An International Rugby Board spokesman said the board and its member unions took concussion extremely seriously.
The IRB had consulted union medics and independent experts as part of research and policy making in this important area, he said.
Former All Blacks flanker Josh Kronfeld, who has had one serious concussion, said commentators shouldn't be making light of such serious injuries.
But he suggested joking about it could be their way of lessening the effect of something which was "quite horrific".
And in regard to players continuing to play after a heavy knock, Kronfeld said: "It's a World Cup scenario and guys are going to do that - it's their one moment in the sun and can make or break a team
"With the same injury in local competitions, like the Super 12 or the European Cup - providing it's not finals time they probably wouldn't play on."
WORLD CUP KNOCKS
48
matches
13
probable concussions
7
players continued
6
taken off
