AFL legend Barry Hall's camp believe they have rattled a "nervous" Sonny Bill Williams ahead of their heavyweight bout in Sydney on Wednesday night.

Things heated up during the pre-fight weigh-ins at Sydney's Darling Harbour when Hall appeared to headbutt Williams as tensions threatened to spill over ahead of their clash in the ring.

Williams, the former All Blacks and NRL superstar, appeared stunned by Hall's aggressive move, and the Australian's camp took notice.

"Sonny didn't know what to do," a member of Hall's team was heard saying after the "headbutt".

After getting briefly separated, the duo calmed down and resumed their staredown before shaking hands.

Williams, who dabbled in a few boxing fights during his code-hopping career, goes in as the slight bookies favourite against the older, lighter Hall.

The 45-year-old Hall weighed in at 97.8kg, giving away more than 11kg to Williams at 109.1kg.

Despite his underdog status, at least on the weighing scale, Hall said he was embracing the "Barry against the world" mindset.

"The press conference yesterday, there was an illustrious line-up, all the names written down, I had two people with me," Hall was quoted as saying on Fox Sports Australia.

"There's a lot of people that don't want me to win this fight, I understand that. There's a lot of money lost if he loses, I understand that also. That's why I need to win by knockout and why I'm going to."

Williams, who is 8-0 in his boxing career, has now dedicated himself to boxing in a final sporting push after retiring from rugby and league.

The 36-year-old enlisted the help of some of the fight world's big names in preparation for his return to the ring, including a six-week training camp in the UK with world heavyweight champ Tyson Fury, Kiwi star Joseph Parker and European champion Tommy McCarthy.

He will also have former WBO middleweight champion and renowned trainer Andy Lee in his corner for the fight.

Williams admitted he was "nervous" ahead of his ninth professional fight "but for me it's a sign that I'm ready to go".

"I've got a lot of respect for Barry, I know he's put himself through the wringer this prep, I don't know too much about his prep but I think he knows all about my prep," Williams said at Monday's press conference.

"I wish we were fighting tonight to tell you the truth but I've got no ill will towards Barry. I hope he goes home to his beautiful wife, beautiful children but I want to smash his face plain and simple.

"There's no glory without putting yourself out there and I'm looking forward to getting out there and just being in the ring with Barry because I know we're going to put on a good show."