Botha, who was beaten on points by Williams on Friday, had tested positive for phentermine, a banned appetite suppressant, the Sydney Morning Herald said in a report.
Williams (6-0) took his unbeaten record to six wins in farcical circumstances when he was awarded the victory over the 44-year-old South African in a bout reduced to 10 rounds from the original 12 at the last minute.
Doubts also surfaced over the fight's status as a purported World Boxing Association (WBA) international heavyweight title bout after local media reported that no officials from the organisation were present.
An incensed Botha, who was well on top in the closing rounds, described the late change of bout length as "match-fixing" and demanded a rematch with Williams, who later said he was more than willing to fight the South African again.
"After, God willing, having a good footy (rugby league) season I'll gladly rematch Mr Botha. Even in South Africa," he said on his Twitter feed.
"Also very humbled by the congratulatory phone call from the WBA on the fight and the win.. To all my supporters nothing but love!!"