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.
"Everyone knows that Christian is an amazing young man, an important member of this team and of the Canberra community.
"Our thoughts and prayers are with Christian and his family at this time."
Lealiifano was born in Auckland and moved with his family to Melbourne when he was seven years old.
He played for the Australian Schoolboys team in 2004 and 2005 before joining the Brumbies rugby academy in 2006. Later that year he was selected for the Australian Under 19 and Under 21 teams, winning the under-19s IRB World Championship in 2006.
He made his Super Rugby debut in 2008 against the Crusaders in Christchurch. Famously his Test debut for the Wallabies lasted less than one minute after he was knocked out against the British and Irish Lions in 2013.
In June he became a father for the first time when his partner Luga Lam-Young gave birth to a boy.