The A$500,000 (NZD$532,000) loan came just before Mundine's recent fight against Danny Green, The Sunday Telegraph in Sydney reported.
There was no written agreement, just a handshake, the newspaper said.
It was the return of a favour, as Mundine gave Williams A$750,000 (NZD$800,000) in 2008 when Williams went to play rugby in France, leaving the Canterbury Bulldogs rugby league team.
"It's very hard to explain our brotherhood in words," Mundine said. "I didn't understand it until I became a Muslim. We trust and we love each other.