Speaking to RNZ's Sunday Morning show, hosted by Jim Mora, to celebrate the 50th anniversary of the band's 1974 number one hit Out on the Street, Riddell said this was the first time he was revealing his diagnosis in public.
"I haven't told many people this but I've had a real battle with cancer, Jim.
"The last two or three years have been a bit tough, but I'm taking a fairly new drug which is doing a pretty good job.
"I am looking at doing another album, I would really love to do another album, I've got lots of songs. I'm working away on that, just quietly and trying to enjoy my life as much as I can. Because I've spent a lot of time - I'm a musician in New Zealand, I've renovated houses - more houses than I should've - to greater or lesser success, and I've worked my bum off really, for not a great deal.
"So I think when you get something like cancer, it does make you sort of straighten up and fly right or try to, because you don't know how long you've got really."
Space Waltz was formed in 1974 by Riddell as a frontman, and it included artists who went on to become distinguished in their own bands; keyboardist Eddie Rayner of Split Enz, and guitarist Greg Clark and drummer Brent Eccles of Citizen Band.
Riddell had twice declined to join Split Enz, upon which Neil Finn did.
"Hindsight is 20/20, isn't it? I mean I probably should've joined.
"They were in the UK at the time and Mike Chunn came back to New Zealand and I was ... watching Neil Finn give a concert.
"And Mike just turned up ... he said come outside I want to talk to you ... he said we'd like you to come to the UK and join the band, and I was just in the throes of finishing my second album.
"There was a mixture of things but I think the thing at the time I really felt I had a commitment to Glyn Tucker who had produced that record and put quite a lot of his own money into it, and of course if I'd gone to the UK I would've walked away from that project."
Last year, Space Waltz reunited for a second album, Victory, which featured new songs as well as re-recordings of selected tracks from their first album.
"We got to number four in the New Zealand charts with the new album ... which is something after all that time.
"You feel like you've been completely forgotten, and you haven't."
The band emerged on the scene in 1974 on New Zealand talent show New Faces, where they performed their original songs.
"We had no idea what it was going to look like. So we watched the show, and the world just blew up. The phone ... literally didn't stop [ringing], every time we put it down, it would ring again for about two hours."
Although they didn't win, the band grabbed attention with their image and sound - with Riddell's voice often compared to David Bowie - and were picked up by a record label for their first album.
But Riddell said his interest in Bowie actually started with a love for crooner Anthony Newley.
"I actually founded a Blues convention in 1969, of all things, when I was about 16.
"I remember when we [Riddell and his friend] heard the first single in New Zealand that Bowie released, which was Space Odyssey, and we were both so excited because this guy sounded like Anthony Newley or we thought he did, I was so excited about it and I mean it really lit a fire under me about David Bowie."
When Bowie's third studio album, The Man Who Sold the World, was released in 1970, Riddell got it delivered to New Zealand using his father's overseas funds.
"Most people thought I was as mad as a hatter. They couldn't see the point... I said at the time, I said I think this is guy is going to be one of the biggest things in world music and it was very hard to get many people to agree but it turned out to be so for him."
Riddell said he never got an explanation for why there wasn't a second album for Space Waltz at the time.
"That [first] album went gold. In fact, it's done more than gold, I think, it's probably done equivalent to New Zealand platinum but I've never been presented with a gold disc for it."
Riddell also helped lay down the rhythm track for Poi E by Pātea Māori Club after being invited on the project by Dalvanius Prime.
