"We violently reject the report," the Communication Workers' Union's Thami Mzileni told reporters in Johannesburg on Tuesday.
"That report is not an honest report because it is not coming from an honest person."
Public Protector Thuli Madonsela's report raised questions about Motsoeneng's matric certificate and said he had received irregular salary increases.
Mzileni accused Madonsela of being a "foot soldier of the white minority".
"Her views are the views that Helen Zille and [Lindiwe] Mazibuko speaks.
"They are the views of the minority. Thuli is a messenger of the Democratic Alliance," he said.
Zille is leader of the DA with Mazibuko the opposition's leader in Parliament.