RNZ reports his Super Rugby club, the ACT Brumbies, announced today that Lealiifano is currently being treated in hospital.
The 28-year-old was a notable absentee from the Wallabies squad for the Rugby Championship named yesterday.
The Australian Rugby Union said it would do whatever it could to support him.
"We were shocked to receive the news of Christian's diagnosis yesterday and our thoughts are firmly with him as he begins his treatment," said chief executive, Bill Pulver.
