To'omua was a shock omission from Rennie's 40-man squad named on Sunday for a three-day training camp on the Gold Coast next month.
Waratahs flyhalf Ben Donaldson and Reds fullback Jock Campbell.were among eight uncapped players named in the squad.
But there was no room for much-capped veteran To'omua, whose Melbourne Rebels are 0-5 following a horror start to the season.
Rennie has left the door well and truly open for To'omua to return ahead of the mid-year home Test series against England, but first he wants to see the 32-year-old produce some strong form for the Rebels.
Adding to his problems though is the fact Rennie sees him as an inside centre but he's been playing five eighth for the Rebels.
"What we know about Matt is he's a really good man, very experienced player," Rennie said.
"He's playing a fair bit of 10 at the moment, and we see him more as a 12. We've got a lot of midfielders who we think are going pretty well as well.
"His job over the next couple of months, 10 weeks, is to put a bit of pressure on us through performance.
"The positive thing is that it's still in his hands. We're not naming a Test squad really until the middle of June.
"He'll be disappointed, but as you'd imagine with him, he's a top man and he's seeing it as a challenge, and he knows he's got a bit of work to do."
Donaldson appeared to injure his calf in the Waratahs' 24-19 win over the Rebels on Saturday night, but Rennie expects the 22-year-old to be fit for the training camp, which starts on April 3.
"We really like what we've seen from Ben so far," Rennie said.
"He's got genuine change of pace and threatens the line."
Former Wallabies great Tim Horan is eager to see young Western Force flyhalf Reesjan Pasitoa rushed into the national squad.
But with James O'Connor, Noah Lolesio and Donaldson named for the upcoming camp, Rennie said there wasn't room for Pasitoa just yet.
"From a pecking order point of view, he's still behind the guys that we've named," Rennie said.
Waratahs forward Jed Holloway, Force speedster Toni Pulu, Brumbies lock Cadeyrn Neville, Waratahs hooker David Porecki, Brumbies lock Nick Frost and scrumhalf Ryan Lonergan round out the uncapped players in the squad.
Pulu qualifies for the Wallabies via his Australian mum, and he played under Rennie at the Chiefs.
"He's very rapid, powerful, he's got a really good skillset," Rennie said.
"He's a very experienced player and there's no reason why he can't step up to international selection."
WALLABIES SQUAD
Allan Alaalatoa, Tom Banks, Angus Bell, Jock Campbell, Ben Donaldson, Folau Fainga'a, Lalakai Foketi, Nick Frost, Jake Gordon, Reece Hodge, Michael Hooper (capt), Jed Holloway, Len Ikitau, Harry Johnson-Holmes, Feleti Kaitu'u, Andrew Kellaway, Rob Leota, Noah Lolesio, Lachlan Lonergan, Ryan Lonergan, Tate McDermott, Fraser McReight, Cadeyrn Neville, James O'Connor, Hunter Paisami, Izaia Perese, Jordan Petaia, Matt Philip, David Porecki, Toni Pulu, Izack Rodda, Pete Samu, James Slipper, Darcy Swain, Lachlan Swinton, Taniela Tupou, Rob Valetini, Nic White, Harry Wilson, Tom Wright.
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