Congress of the People (Cope) deputy leader Mbhazima Shilowa on Thursday accused KPMG of colluding with party leader Mosiuoa Lekota in its probe into the party's finances.
He has also referred the report to his lawyers for possible legal action.
KPMG has declined to comment.
"The flawed KPMG report could not find any evidence of Cope president Mr Patrick Lekota's allegations of R20-million embezzlement," Shilowa said in a statement.
"However, the manner in which the investigation was conducted was regrettable and unfortunate. While it was to be expected that Mr Lekota would use underhanded tactics and dubious methods to achieve his mission, KPMG's collusion has been more astonishing," Shilowa alleged.
At the outset, an "unflattering" preliminary report was issued at least a week before his interview by the investigators.
"Furthermore, the packaging of the report demonstrated beyond any doubt that a particular outcome had been dictated to the investigators."
Shilowa gives report to lawyers
Shilowa said the report had been referred to his legal representatives for advice on possible defamation action, and on how to deal with KPMG's "unethical conduct."
"The way this investigation has be mishandled confirms the view that this was a witch-hunt aimed at [the] victimisation of those who oppose Mr Lekota's election as an inaugural elected president of the party," he said.
The Cape Times reported on Thursday the report confirmed Cope had "misrepresented" its financial statements to Parliament.
When approached for comment, KPMG responded it was a confidential client matter and it was not company policy to comment on such to the media.
Cope spokesman Philip Dexter was not immediately available for comment. — Sapa