"When I saw the size of my stomach it was a real wake-up call. I looked pregnant.
"I was pissed off about it at first, because it was like having someone photograph you on the toilet.
"But in the end it did me good because it forced me to get some beef off.
"I was staying next to a New World supermarket in Auckland and it was too easy to nip out and stock up on my favourite Malborough wines.
"When I stopped drinking so much of that stuff, the weight just fell off.
"It dawned on me that I am in my mid-50s now and I can't just keep partying like I used to with no consequences."
Campbell enjoyed a career boost when he was a judge on the New Zealand's Got Talent television show, with Rachel Hunter and Jason Kerrison.
But he has ruled out a repeat performance.
"I had a great time doing that show, but that is because I didn't take it seriously," he said. "It is nothing to do with finding talent but is all about producing half an hour of entertaining television.
"Some of the kids on the show were great and I got on really well with the other judges but when it was all over I thought, stuff that.
"Anyway, I think the pictures of me with the spliff didn't help my chances of being asked back."
Campbell is involved in a legal dispute with other previous UB40 members who have been touring Europe under the same name.
At the Raggamuffin concert, Campbell will be joined by original members Mickey Virtue and Astro.
"The other UB40 is a headless chicken, a completely rudderless ship," he said.
"I am the original UB40 singer and writer so if anyone wants to see the real band, come and see us."
Campbell promises a mix of UB40 classics such as Red Red Wine and Kingston Town plus tracks from their new album Silhouette at Raggamuffin.
He can't wait to give Kiwi fans a taste of reggae as other major acts on the bill include American hip-hop legends Ice Cube and Cypress Hill.
"I'm not sure why hip-hop acts keep getting booked for a reggae music festival, but we will be there to balance that up."
