Christian Lealiifano has been diagnosed with Leukaemia.
The 28-year-old Wallabies star, who was born in New Zealand, had been suffering from a brief illness according to information released on the Twitter feed of his Super Rugby club the Brumbies, before the serious condition was identified.
Brumbies interim Chief Executive, Phil Thomson, said the focus was on ensuring Lealiifano received the best care and support possible.
"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.