Toomua will play his first Test in almost two months when the Wallabies play Ireland in Dublin on Sunday morning Australian time.

The hard-hitting defender was knocked out in Tests against South Africa and Argentina and can't remember passing a cognitive test to get back on the field.

But officials stepped in and recognised Toomua was still suffering symptoms, preventing him from playing on and ordering him to take a month off.

Brumbies teammate Christian Lealiifano stepped up in his absence and Toomua admitted he thought he might not get another chance to play for Australia this year.

But new coach Michael Cheika has given him a chance and Toomua says he won't temper his physical approach to the game.

"I haven't thought [about concussion], it was the right call that I sat out … there are certain areas of my technique I probably have to change a little bit and I've worked hard," Toomua said.

"But it's not something that's on my mind, pardon the pun. Quite often I go high and expose my jaw, that's what happened in Argentina.

"It's organised chaos out there so while you've got an idea of how you want to defend, sometimes it doesn't work out that way. It's a balance.

"You've got to make sure you're trying to do the right thing, physicality is just a natural consequence of playing rugby."

Toomua has made big tackles one of his biggest weapons, using his strong defence to put opposition playmakers under pressure and force them into making bad decisions.

He replaces Christian Lealiifano in the starting side, as Cheika tinkers with the Test line-up in an attempt to broaden his knowledge of individual players.

Winger Henry Speight will make his debut while Luke Jones gets his chance in the starting XV and Kurtley Beale joins the bench.

"I don't think in any way that by changing a few lads in here that we've weakened the team at all," Cheika said.

"It's so level and without being too naive, the choices have been made on the flip of a coin. There are some tight battles in there for selection and while I'm interested to see all of the players on tour, I'm in the competitive environment."

Toomua had cemented his place as the Wallabies' inside centre over the past year before he was ruled out because of concussion and Lealiifano stepped up.

Last year it was a role reversal on the spring tour with Toomua taking his opportunity when Lealiifano got injured.

The pair are in a battle to stake their claims before the World Cup next year with Beale also pushing for the No. 12 jersey.

"[Not playing again this year] went through my head, it was a sense of irony with how it worked out with [Lealiifano]," Toomua said.

"At the end of the day I've got a chance now and I just want to do as well as I can. Christian's done a good job and now it's a chance to show what I can do."