Despite the jokes, Kirwan told the audience Leitch "was always there for my dad", who had a heart attack.
"I know we are roasting him, and he treated me pretty poorly in the early days - and he ruined most butchers in Auckland - but I consider him a special New Zealander."
The pair hugged after Kirwan spoke, with Leitch saying, "John for f**** sake get over it."
All proceeds from the event will support the Prostate Cancer Foundation and Auckland's Westpac Rescue Helicopter.
Former Minister of justice, Simon Power was also among guests.
At age 65, Leitch announced he was battling bladder cancer. He is now in remission.
The Great Charity Roast coincides with the foundation's Blue September appeal, which aims to highlight the dangers of the disease that kills 600 Kiwi men every year.
Because the foundation receives no government funding, the annual campaign is essential to its work supporting prostate cancer sufferers.
As well as the roasting, attendees can participate in an auction.
Among the experiences to be sold, will be an opportunity for up to six people to have a private drink with Prime Minister John Key.
The Mad Butcher
• Peter Charles Leitch, 72, opened his first butchery in Mangere East in 1971.
• His nickname came about after an incident in a pub where he was called "that f***ing mad butcher" by a patron, and he decided to use it in a radio promotion advertising his shop.
• Leitch is an avid Warriors supporter, ambassador for Allergy New Zealand and the Prostate Foundation, and has sponsored Middlemore Hospital's national burns unit.
• He was awarded the Queen's Service Medal in 1991, and was made a Knight Companion of the New Zealand Order of Merit in 2010.
• At age 65, he announced he was battling bladder cancer. He is now in remission.
The Great Charity Roast
• Sir Peter Leitch the subject
• Proceeds: To the Prostate Cancer Foundation and Auckland's Rescue Helicopter Trust.