George Kambosos Jr has labelled American Devin Haney a "rat" ahead of their fight to become undisputed lightweight boxing world champion.

The pair came face-to-face for the first time in Melbourne before Sunday's showdown, which is expected to draw more than 50,000 spectators to Docklands Stadium.

Kambosos (20-0) owns the IBF, WBA, WBO, and The Ring Magazine belts, while Haney (27-0) has the WBC title.

It is the Australian's first fight on home soil since 2017, having set himself on the world title path overseas.

Kambosos claimed Haney helped him prepare for his upset victory over another American, Teófimo López, in New York last November by sending him messages with details about the López camp.

"This guy is an informant," said Kambosos, who said he still had the messages and could make them public.

"This guy is a rat … in the Teófimo López fight, against a foreigner, he was messaging me, telling me all the details: 'Teo at the hookah lounge'; 'Teo having problems with his wife'.

"Your own countryman, you were going behind his back so I'm going to whip your ass and when you get back to Las Vegas, you're going to be on the lookout for Teófimo López, because he's going to be looking for you too."

Haney did not deny he sent information to Kambosos.

The 23-year-old seemed bemused by the Australian's antics, saying he thought it was an "act".

"When is the act gonna stop?" Haney said.

"We can see right through it. Just be you, that's all you got to do — be you. You're acting."

Kambosos replied: "I'd rather be an actor than a rat."

Haney arrived in Melbourne from Las Vegas without either of his trainers — his father Bill and Ben Davison — who failed to secure travel visas due to historic offences.

He said he would not let the setback stop him becoming undisputed champion and declared he would outclass Kambosos.

"There's nothing that he can do in the ring that is better than me," Haney said.

"I think he's a good fighter but I think I'm on a whole different level."

ABC Sport will have a live blog of the Kambosos vs Haney fight and the full undercard on Sunday.

AAP