"My career in the near future is in Australia with the Wallabies, but I can't say enough how much I've appreciated the support of the Leinster public and the team at Leinster."
Commenting on Elsom's departure, Leinster coach Michael Cheika said: "Rocky really wanted to stay at Leinster and also represent his country in Test match rugby - which is the highest honour for any player.
"Up until today we had been negotiating with the Australian Rugby Union (ARU) to secure this scenario but unfortunately the ARU would not allow it and told him that to play for the Wallabies he had to be playing with an Australian franchise.
"Rocky's desire to play Test rugby has to be respected and we wish him well with that and thank him for his contribution to the club and all its supporters.
"We hope to see him back in the Leinster blue one day."