Smith (60) ended a 20-year association with the All Blacks at the end of The Rugby Championship in October last year.
However, he explained that he was aware of his illness throughout that farewell tournament after an MRI scan "showed up some tumours".
Smith told Fairfax the pathology report on the prostate had been completed, and it indicated that, most probably, he had been cured.
Smith shared his story with Fairfax to raise awareness, and said his local doctor in Cambridge, Waikato, had detected abnormalities.
Since he turned 50, he had been having an annual medical check-up, and last year his doctor started doing PSA tests while he was doing blood tests.
PSA stands for Prostate Specific Antigen, a protein produced by the prostate. Changing levels can be a sign there are cancer cells in the prostate.
"I was getting the blood tests about every six months. It was high PSA levels that initially alerted the doctor," Smith told Fairfax. "High levels don't automatically mean you've got cancer, but it is an alert, and it led to me getting a digital exam, and he then thought, 'Hmm..there's a hard spot there.' Could be nothing, but he sent me to the urologist.
"Then I had an MRI test (magnetic imaging scans), which showed up some tumours."
"Through the Rugby Championship I knew I had it, but I didn't really talk to anyone (inside the All Blacks). The only one was the team doctor, Tony Page, and I didn't even tell him.
Smith decided to 'just get the thing out' and before the operation came a CT scan to see if the cancer had spread elsewhere.
"The results took a week to come through, and I was starting to picture, 'Jeez, are there that many on the screen that they're still counting them? Have I got it everywhere?'
"Then the text came through: 'You're all clear, apart from the prostate.' It was as simple as that. The relief was enormous."
The discovery he had cancer played no role in Smith's decision to step down from the All Black coaching team.
Nicknamed "The Professor", Smith had a 20-year association with the All Blacks as a player and then in three spells as a coach.
Smith's next move has been a subject of huge intrigue around the rugby world, and he is intent on fulfilling "a couple of little projects I started arranging immediately".
"I'm going to help the Kobe Steelers club in Japan," Smith said. "They have a relationship with the Chiefs, and I'm going to have an involvement with the coaches at Kobe and get a bit of coaching in from time to time.
"I'll also be doing two to three weeks with the Italian national team coaches over the year. I'll attend a couple of camps to have a mentoring-type role with Conor O'Shea and Mike Catt. I have a real passion for Italy, its people and their rugby, so Trish and I are looking forward to getting back there now and again."