Mr Rennie confirmed the person "strongly suspected" of the leak of Cabinet papers to Labour foreign affairs spokesman Phil Goff was a former Labour Party researcher employed on a short-term contract .
The report also found the commission failed to guard against the risks posed by having a former political staffer handle sensitive documents.
While the leak was previously linked to that clerical assistant, the report also savaged two senior MFAT employees, including an ambassador for publicly airing their concerns about the restructuring.
The 18-month $510,000 investigation by Paula Rebstock was "unable to find definitive evidence of who was responsible for the unauthorised disclosure of the Cabinet papers".
However, Ms Rebstock reported "a strong suspicion" that the leak of the Cabinet papers was made by a temporary staff member working at the commission.
"I am extremely saddened and disappointed by this," Mr Rennie told reporters today.
That person identified by Ms Rebstock took legal action in a bid to prevent her suspicions from being reported to Mr Rennie and a High Court suppression order prevents him from being identified.
The person had previously worked in the Labour Party research unit, Mr Rennie said.
Ms Rebstock's report found the commission assigned the person to handle sensitive government documents "without mitigating possible risks due to the person's prior work for a political party; likely lack of experience of following Cabinet guidelines for handling Cabinet papers; and dual role working the secure area of the Commissioner's Office and in the more public area of SSC."
While the hiring of people who'd previously worked for political parties was not problematic, "it is the department's responsibility to induct, train, and provide oversight to mitigate any risk".
Meanwhile, the report also found the behaviour of some managers at MFAT during the restructuring "fell below the standards expected of people in their position", by creating a perception within the department that it was acceptable to air their opposition to the changes publicly an allow it to be used for "political purposes".
Ms Rebstock also found it "probable" that some senior MFAT staff leaked a variety of confidential material to Mr Goff and to the media.
Mr Rennie said he considered whether the three people involved should be publicly identified but concluded it wasn't in the public interest due to the further litigation and expenditure of public money that would entail.
He was unaware of any disciplinary action taken against the MFAT staff identified as probably leaking material to Mr Goff. The pair include a Head of Mission or ambassador.
Foreign Minister Murray McCully said the report suggested a number of officials were placed under pressure from Mr Goff to release information that they were not entitled to disclose.
"It also records that a former researcher from the parliamentary offices of the Labour Party found employment at the State Services Commission, had possession of the leaked Cabinet papers and was unable to satisfactorily explain the scanning of documents that exactly matched the size of the two papers. I believe the public can draw their own conclusions both as to the actions of this individual and the role played by Mr Goff."
But Mr Goff told the Herald he did not know the identity, background or even gender of the person who supplied the Cabinet Papers. He freely acknowledged ringing MFAT staff for information about the restructuring but said as the Opposition foreign affairs spokesman, he had no power to pressure them.
He described the trio fingered by Ms Rebstock as "whistleblowers" who had acted in the public interest to reveal the restructuring that harmed New Zealand's diplomatic service .
"We lost some of the most skilled and experienced diplomats who had served successive governments loyally and professionally," he said.
"The inquiry should have been into how the Government managed to get the restructuring so wrong. Even Foreign Affairs Minister Murray McCully was forced to admit the job had been botched and blamed chief executive John Allen."
- By Adam Bennett of the New Zealand Herald
