Tomkinson (21) will line up at second five-eighth for the Highlanders as they take on the Waratahs in Sydney tomorrow night.
Tomkinson has been tipped by many as one to watch even when he was at high school and was training with the Highlanders a year after leaving Otago Boys' High School.
This will be his eighth appearance for the Highlanders but the first in his preferred appearance at second five-eighth.
His two other starts were both on the wing.
Tomkinson comes into the team in place of Tei Walden, who had concussion symptoms earlier this week.
Walden played golf last Sunday following the bruising game against the Lions the night before and started felling groggy near the end of the round.
Highlanders coach Aaron Mauger said Walden had been feeling fine over the past couple of days but he had not trained in the earlier part of the week.
Mauger was looking forward to unleashing Tomkinson.
"Sio had an awesome off-season and was travelling really well but unfortunately got an injury at training and he has come back and performed really well for the Bravehearts and in his club footy," Mauger said.
"He has taken his opportunities when he has been out on the field for the Landers. It is a really exciting opportunity for him.
"He brings a real physical dimension to his game - that is how he likes to play the game. With him combined with Robbie [Thompson] that will bring plenty of power to the midfield."
Aaron Smith and Dillon Hunt come back into the side while Ash Dixon gets a start at hooker. Lock Jackson Hemopo comes back into the starting side for Alex Ainley.
Mauger said openside flanker James Lentjes had hurt his ribs last week and had not been able to train earlier in the week.
Tevita Li is over his foot injury and gets a sport on the reserves bench.
Mauger said the Waratahs were fighting fit and would be hurting after the tough loss against the Crusaders last week.
"That first-half performance against the Crusaders showed how dangerous they could be ... they showed the threat is there. We are expecting them to be at their best and we have plans in place to combat that."
Although the side is travelling again - it will have the match-day squad of 23 plus two others in Australia - it did have the benefit of having a solid week of training.
"We are back to a full week and had a bit more time to get into some detail and get some growth around a few parts of our game. We've had short weeks with travel and that. We are in good space to jump on the plane."
Highlanders v Waratahs line-ups
Sydney, tomorrow, 9.45pm
Highlanders: Ben Smith (co-captain), Waisake Naholo, Rob Thompson, Sio Tomkinson, Tevita Nabura, Lima Sopoaga, Aaron Smith, Luke Whitelock, Dillon Hunt, Elliot Dixon, Tom Franklin, Jackson Hemopo, Tyrel Lomax, Ash Dixon (co-captain), Daniel Lienert-Brown. Reserves: Liam Coltman, Aki Seiuli, Kalolo Tuiloma,
Shannon Frizell, Marino Mikaele-Tu’u, Kayne Hammington, Josh Ioane, Tevita Li.
Waratahs: Israel Folau, Cam Clark, Curtis Rona, Kurtley Beale, Taqele Naiyaravoro, Bernard Foley, Jake Gordon, Michael Wells, Michael Hooper (capt), Will Miller, Rob Simmons, Tom Staniforth, Sekope Kepu, Damien Fitzpatrick, Tom Robertson. Reserves: Tolu Latu, Harry Johnson-Holmes, Shambeckler Vui, Ryan McCauley, Jed Holloway, Nick Phipps, Lalakai Foketi, Bryce Hegarty.