Johnson, who will turn 36 on Friday, had no hesitation when he heard about the cause.
"I was a late replacement and have only been training for five weeks. It's such a good cause and during my school days back in the States I lost a couple of close classmates to suicide. It's a sad thing and so many people have a story to tell about suicide," Johnson said.
A product of Chipola Junior College in Maryland, Florida, and the University of Cincinnati, Johnson, has played for the Auckland Stars, Wellington Saints and the Hawks in New Zealand.
He played for the Shawn Dennis-coached 2006 Hawks side which won the NBL title before plying his trade in Switzerland.
Johnson heaped kudos on the work Napier Boxing Club trainers Rod Langdon and Ryan Whittaker contributed during the 15 to 20 hours training he completed each week for the three two minute round fight.
"I'm seven centimetres taller than Tamati and I've got a longer reach which I intend to use to my advantage. The plan is to keep my distance and work it," he added.
Auckland-based Kemp, 34, will also be making his boxing debut. The former Eskview club rugby player also lost schoolmates to suicide.
"While I've been training for three months I haven't been able to do it as often as Kareem so I'm picking we will have put in a similar amount of sparring and training by the time we step into the ring on Saturday night.
"I met Kareem for the first time at the weekend and he's s nice guy. It's a shame I'm going to have to punch him in the face a few times but it's what you have to do," Kemp said.
An insurance adviser for the event's major sponsor, Partners Life, Kemp keeps reasonably fit by playing indoor and outdoor cricket.
His Warkworth senior grade team has had a memorable start to the summer and he is hoping this proves to be a good omen for him on Saturday night.
Depending on his level of success in the ring Kemp may take on more charity fights in the future.
"You never say never but I will stick to this one for now. I still don't know how the nerves are going to be on Saturday. I'm flying down on the day so at least I won't have all day for the nerves to take over," he added.
While the Kemp-Johnson clash is the feature male bout, the feature women's fight will see long-serving Hawke's Bay Tuis rugby halfback Gemma Woods take on Osana Leutele, a sister of Hawke's Bay's 2011 National Youth Super Heavyweight champion Kiki Toa Leutele.
All 55 corporate tables have been sold but general admission tickets are still available from Ticketek or the Napier Municipal Theatre.
The card
Red Corner Blue Corner
Kelly Bray v Rebecca "Blue Belle" Winikerei
Heath Clark v Simon "The Ice Man" McFarland
Sarah "Killer" Smith v Gemma "Fighting Irish" Hyde
Shane Smith v Fiva Latu
Stacy "The Beast" Bateman v Aurora Akauola
Shane "Hong Kong" Lui v Ryan Van Zonneveld
Gina "Celtic Thunder" Graham v Cherylene "Chera Da Hero Hurae" Lawrence
Rakei Ngaia v Willie "The Terminator" Tran
George Diack v Leeroy Christison
Sharyn "The Shizzle" Scurr v Laura-Jayne Braithwaite
Allan "The Annihilator" Smith v Andrew "Squids" Urquhart
Lisa "Lethal"Longstaff v Georgia Kirby
Stacey "Mad Dog" Mizzi v Te "Mr T" Hunt
Gemma Woods v Osana Leutele
Tamati "Too Tense" Kemp v Kareem "The Dream" Johnson