The Department of Public Works has finally broken its silence on the letter that allegedly proved President Jacob Zuma knew about the costs of upgrades to his Nkandla homestead in KwaZulu-Natal.

On Sunday, Public Works Minister Thulas Nxesi said that Zuma was not informed of costs to upgrade security features at the home during the planning stages.

"President Zuma is not involved in this process whatsoever," Nxesi told journalists at a special media briefing in Pretoria.

The report revealed that R71-million was spent directly on security, while a further R135-million was spent on operational costs incurred by state departments involved in the upgrade.

Nxesi said operational costs included the running of the clinic, helipad and accommodation for police and the South African National Defence Force, but said further details could not be revealed, citing security reasons.

He told Talk Radio 702 on Monday that the 15 service providers involved were under investigation for possible cost irregularities regarding the operational costs.

"We do not want to jeopardise anything, because we have indicated that if there was collusion. We are going to take legal action. Therefore we cannot start disclosing people's names in the public."

On Monday, the minister's legal advisor, Phillip Masilo, said that Public Works had never seen the letter that was apparently sent by then-minister Gwen Mahlangu-Nkabinde, detailing the costs of the upgrades.

But after Eyewitness News sent him a copy, Masilo said the letter's authenticity was dubious.

"It's not clear whether this letter is authentic, whether this letter was sent to the President or whether there was any confirmation of receipt. It is the first time I see that internal memo."

Masilo said such a memo would never be sent in that format to the president, and that there was apparently no record of the letter at the department.

But Masilo also said that proving whether or not the letter was authentic was not a priority.

"There is so much to deal with at Public Works that a letter like this would be a serious waste of time on our part."

He said such matters should be referred to the police.

Masilo said the journalists who received the letter should come forward with the person who leaked it, as it may be a breach of the president's security.