Suaalii was playing for the Kings School's first XV side at 14 years of age. He also represented the New South Wales Schoolboys Sevens and Australian under 18s Sevens teams before his 16th birthday.
Wallabies coach Dave Rennie reportedly offered the 197cm giant a three-year contract with the New South Wales Waratahs when he turned 18.
Rabbitohs CEO Blake Solly is not allowed to sign Suaalii until August, but is eager for the opportunity to lock him in.
"He's not just a very good player; he's a great kid from a beautiful family," Solly said.
"He's just the sort of young person you want at your club."
Rabbitohs General Manager of Football Shane Richardson touted Suaalii as a future star in February 2019.
Rabbitohs General Manager Shane Richardson. Photo / Getty
"Joseph has the body shape and skill levels that extend well beyond his years, but most importantly, he has a desire to achieve everything he can in rugby league through hard work and persistence," Richardson said.
"It's exciting to see young talent like Joseph in the red and green, showing the Rabbitohs are the club of choice for young talent in our game and young players from other codes, and I'm sure our members will also enjoy watching them progress through the years."
First-grade players must be 18 years of age, meaning Suaalii is not eligible to make his NRL debut until August next year.
Former South Sydney star Craig Wing is adamant Suaalii will live up to expectations, and described him as a combination of Sonny Bill Williams and Greg Inglis.
"The thing that really stands out with Joseph is he's got the physique of a 19-year-old already," Wing said in February.
"When he moves he's really well balanced, he's got a great running technique and he glides across the ground.
Craig Wing. Photo / Photosport
"If I was to sum him up I would say he's like a combination of Sonny Bill Williams and Greg Inglis, if you put those two together.
