Brumbies centre Christian Lealiifano has been diagnosed with leukaemia, the Super Rugby outfit announced on Saturday.
Following investigations after a brief illness, it was found Lealiifano, 28, is suffering from the disease.
"At the moment it's just a matter of ensuring Christian receives the best care and support he can at this time," said Brumbies interim chief executive Phil Thomson.
"He is one of the most respected players within this group and this news has come as a huge shock to all of our players, coaches and staff.
"Everyone knows that Christian is an amazing young man, an important member of this team and of the Canberra community.
NEWS: It is with great sadness that we inform fans that Christian Lealiifano has been diagnosed with Leukaemia. pic.twitter.com/pkr9Fuf2Dk-- Brumbies (@BrumbiesRugby) August 6, 2016
"Our thoughts and prayers are with Christian and his family at this time.
"We ask that the Lealiifano family's privacy is respected and no more comments will be made at this time."
Lealiifano is an Australia international, having represented the Wallabies 19 times since 2013.
Australian Rugby Union (ARU) CEO Bill Pulver added: "We were shocked to receive the news of Christian's diagnosis yesterday and our thoughts are firmly with him as he begins his treatment.
"Anyone who has ever had any involvement with Christian knows he is an outstanding young man, as well as being an exceptionally talented Rugby player, and the ARU will do whatever it can to support Christian and his family now and throughout his treatment.
"I know that everyone involved in our game will rally behind Christian and his Wallabies and Brumbies teammates through this difficult time."
He is one of the longest-serving Brumbies players after making his debut with the Canberra-based side in 2008.
"The Brumbies primary concern is for Christian's health and any further updates on his progress will come via the club and with his personal approval," a Brumbies statement read.