Numsa general secretary Irvin Jim said to reporters: "It's shocking that we are told it is classified.
"It must be declassified because we need to know who is right and who is wrong."
The report is believed to shed light on how money was spent on refurbishing President Jacob Zuma's private home at Nkandla in KwaZulu-Natal.
It was classified following an investigation into the matter.
Jim said the union found it problematic that the department tasked with investigating the matter -- public works -- had come back claiming the information it had obtained was classified.
"We are told that the initial expenditure, which was supposed to have followed proper procedures, was about R27m, but currently it stands at R206m, meaning you offered companies tenders and then embarked on an escalation. We should know why," said Jim.
"A dark cloud of suspicion of corruption and looting of the public purse hangs over this whole matter and only through an expeditious and transparent process will [we] settle or resolve the matter."