After putting pen to paper on his new deal, Lealiifano said he was ecstatic to secure his future with the Canberra-based Super 14 club.
"I've loved being part of the Brumbies program," Lealiifano said.
"It's been great for my rugby to be able to train alongside players like (Stephen) Larkham, (Stirling) Mortlock and George Smith and each week I've picked up new things and my game's continued to grow."
"I've really enjoyed my opportunity to play Super 14 this year and I'm keen to continue building on that."
Brumbies coach Laurie Fisher believes Lealiifano and Toomua will provide invaluable depth in a key position for many years to come.
"Christian has risen to the challenge of playing Super 14 rugby this year, he's benefited from each match and he's a player who's committed to taking his game forward with the Brumbies," Fisher said.
"While Matt has only had limited playing opportunities so far, for an 18-year-old he has a very mature approach to his football and is a player capable of playing regular Super 14 rugby."
Lealiifano will start for the Brumbies in Friday's Super 14 encounter against the Chiefs which will also see Australia duo Adam Ashley-Cooper and Stirling Mortlock return to the side.
Mortlock, who underwent shoulder surgery after the Rugby World Cup last year, will line-up at outside-centre, while the versatile Ashley-Cooper will start at full-back in just his second appearance of the campaign after an ankle injury.
Fisher is hoping the experienced pair will be able to spark the Brumbies into action as they make a run at the Super 14 semi-finals.
"It's pleasing to have these players returning from injury and it's no secret that you'd rather have your Wallabies on the field than on the sideline," Fisher added.
"Both Stirling and Coopy are strong ball runners, they are very direct with the football and they'll give us a few more attacking options."