The legendary lock, who turned 80 in June, was diagnosed with the disease on Tuesday after six months of ill-health.
Last month Sir Colin was admitted to Waikato Hospital over concerns with a kidney illness.
Sir Colin said the diagnosis has been "tough" on his family.
"It's bloody hard for Verna, the kids and grandkids."
Sir Colin's son, Glynn, says they don't know if the cancer is terminal or not as his father needs to go back for further tests at Waikato Hospital.
He said he'd had a tough time in Waikato Hospital and had to be put on a diet as his kidney wasn't functioning properly - not flushing and instead was making him retain water.
The news about his father was a big shock and the family expected a tough road ahead. However, they were now hoping to get on and deal with the news privately.
"We're taking some time to sort of deal with it, really. We just want to be left alone to deal with it."
Sir Colin's sister-in-law Beryl Meads told the Herald she expected him to put up a big fight, she said.
Given he was so well-known, he was also popular in the hospital ward but he had been too tired to handle all the visitors.
"He was in the hospital and had his name on the door and other patients were coming in to meet him.
"He was very tired and I'm saying 'he's not there for a holiday' and they finally took his name off the door. Then they put 'Mr M'. Well, nobody was fooled by that."
She has been fielding calls from wellwishers, including his former teammates, from all around the world, she says.
She said whatever happened, the outcome was not good.
"Logic says somethings thing you can't change, but with this, there's no getting away from what's happening now."
Sir Colin today paid tribute to the medical staff in his hometown and at Waikato Hospital for their exemplary care.
"The care and help I've had from the doctors and nurses at Te Kuiti Medical Centre and hospital have been wonderful, and we as a family, appreciate it.
"We are very lucky indeed to have such a great facility in our town.
"I was in Waikato Hospital too for a few weeks and everyone up there was the same - they couldn't do enough for me."
Sir Colin said he's been inundated with get-well messages from New Zealand and overseas and thanked everyone for their concern.
"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.
"But for now, it's about me fighting this, and we want to get on with what we are facing privately, as a family."
Today New Zealand Rugby chief executive Steve Tew offered the support of everyone involved in rugby to Sir Colin and his family.
"Sir Colin remains a true legend of the game and it is with great sadness we hear of the seriousness of his condition. Our thoughts, like so many others in New Zealand and around the rugby world are with him, his wife Verna and his family as he works through the next steps in his treatment.
"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."
Friend and rugby manager Phil Kingsley Jones, who had organised speaking tours to the United Kingdom with Sir Colin in recent years, was upset to hear about the latest health battle.
"I'm so sad. As I grew up he was one of my heroes and he's still my biggest hero," he said, as the shock of the dreadful news sank in.
His thoughts went to Lady Verna at this difficult time who supported her husband going on overseas in recent years even when her own health was failing.
Former All Black Josh Kronfeld said he was devastated by the news about a man who did not shy away from being one of the most recognised players of the 20th century.
Kronfeld said "tree" had a special gift of being able to interact with everybody.
In his hometown of Te Kuiti, Waitomo Mayor Brian Hanna said Sir Colin wasn't the type to back away from even this kind of challenge.
Hanna said it would be wrong to underestimate the legendary All Black great's strength and he was sure to face the battle head-on with the full support of the community.
