The 28-year-old centre will feature for the Wallabies in the Rugby Championship this autumn, but will still be available for Leicester during the rest periods.
They coincide with rounds one and four of the new Premiership season, and he will then be available for the remainder of the English domestic campaign.
Toomua will then join the Rebels for the 2019 Super Rugby season, and although he is looking forward to the move, he admits it was a tough decision to make.
"It has been a tough couple of weeks and one of the hardest decisions I've had to make in my career," he said.
"However, the opportunity to return home to my wife and family after almost three years apart and, potentially, play in another World Cup was too hard to ignore.
"I absolutely love the club, have become close friends with all of my team-mates and want to go out in a successful way."