Tomkinson was shown a red card in the dying minutes of the match between Otago and Auckland at Forsyth Barr Stadium on Saturday.
Tomkinson went in hard to tackle Auckland replacement back Simon Hickey and his shoulder collided with the jaw of Hickey. The Aucklander went down like a sack of spuds.
Hickey appeared to be out cold and lay on the ground.
After reviewing the footage on the big screen, referee James Munro issued a red card.
Tomkinson is a rugged defender and hits hard.
He picked up a yellow card for the Highlanders in its match against the Brumbies in February this year for a tough tackle on Brumbies fullback Tom Banks. He was then given a yellow card when playing for the Highlanders against the Crusaders in Christchurch last month after colliding with prop Oliver Jager.
Now, with a red card, Tomkinson may be looking at missing a few weeks for what was a dangerous tackle.
Otago coach Tom Donnelly said Tomkinson got that tackle wrong and he may be looking at some time on the sideline.
The aggressive nature of his game, including the tackling, was not something he wanted to take away from Tomkinson.
A judicial hearing is expected to take place sometime this week.