Sir Colin Meads has this week been diagnosed with pancreatic cancer, after six months of ill health.
"I've been crook for the past six months, and having all sorts of tests," Sir Colin said in a statement.
The 80-year-old was admitted to Waikato Hospital last month with kidney concerns, and said he's been inundated with get-well messages from around the country and overseas.
"Thanks to all the people from all over the world for the cards and well-wishes, but especially to our great friends locally - their support has been immense," said Sir Colin.
He said the diagnosis has been hard on his family, but they are supporting each other through his "toughest test of all time".
"It's bloody hard for [my wife] Verna, the kids and grandkids," he said.
"We'll come right together. For me now, it's about fighting this and we want to get on with what we are facing privately, as a family."
"Sir Colin remains a true legend of the game and it is with great sadness we hear of the seriousness of his condition," said New Zealand Rugby CEO Steve Tew.
"We knew Sir Colin had been ill for some time, however the latest update on his condition is concerning to us all, but we remain hopeful that Sir Colin's strength and determination will help him through."
Sir Colin, nicknamed 'Pinetree', played 55 Test matches for the All Blacks between 1957 and 1971, and is considered among the world's greatest rugby players.