Jeff Fenech, who at one time was the trainer of former undisputed heavyweight champion Mike Tyson, believes the boxer would seriously hurt Sonny Bill Williams or Paul Gallen.

On Wednesday, Australian promoter Brian Amatruda placed $1 million on the table to get Tyson to Australia for an exhibition fight against Williams, Paul Gallen or Barry Hall. There are now reports that the offer may have been upped to $3 million.

Tyson, 53-years-old and retired since 2005, has been actively training and recently discussed the idea of returning to the ring to take part in exhibition bouts to raise money for charity.

Fenech, who is still very close to Tyson, says the retired boxer would be a real danger to the health of Williams or Gallen - who both made their names in rugby and tested the waters in boxing.

The veteran trainer can't see a scenario where Tyson faces either man.

"Great publicity, but it's not going to happen," Fenech said to The Age. "They're offering him half a million or a million dollars - he could get more than $20 million to do it in Saudi Arabia. Why would he come here?

"Let me tell you something. If he hits Sonny Bill Williams or Paul Gallen, he will be up for manslaughter. I'm serious. If he punched Paul Gallen, he could kill him. These guys have never been punched by somebody like Mike.

"You see what he's doing to the pads, imagine Mike hitting one of those guys with one of those uppercuts. More than likely they will be in the morgue the next day. Fighting Sonny Bill? Who has Sonny Bill fought? Sonny Bill won't even fight Paul Gallen because I don't believe he can beat him."

But Fenech does believe Tyson would return to the ring, in an exhibition, for charity.

"He will do it, probably for charity, it will happen. One million percent," Fenech said.

"It's an exhibition. Mike doesn't want to hurt anybody and nobody wants to hurt Mike. Mike is 53 years of age, he doesn't want to get punched in the head. He is happy to do something for charity and do an exhibition."