Ms Rebstock was last year asked to investigate the leak of Cabinet papers about cuts to the Ministry of Foreign Affairs and Trade's staff.
Suspicion fell on A after records at his place of employment showed that on May 1 last year he scanned documents with the same number of pages as Cabinet papers shown to media the next day by Labour's foreign affairs spokesman, Phil Goff.
The papers detailed an embarrassing backdown on restructuring plans which were opposed by diplomats.
During an interview with Ms Rebstock via Skype, the public servant did not recall scanning the papers but Ms Rebstock observed he had difficulty answering questions.
In a letter late last year, Applicant A was told Ms Rebstock had reason to believe that it was more likely than not that he was responsible for the leak.
Applicant A went to the Court of Appeal after failing to persuade the High Court to suppress parts of the report dealing with his alleged involvement.
Lawyer Jason McHerron yesterday told the judges suspicion was not sufficient basis to blame his client.
Justice Harrison doubted whether there was anything wrong with Ms Rebstock setting out accepted facts and making a finding based on them.
Justice Mark O'Regan said Ms Rebstock's draft report contained "ample evidence to make the finding she made".
For the Crown, Deputy Solicitor-General Una Jagose said in her submissions the investigation had been conducted in a way that was scrupulously fair to Applicant A.
"There really is nothing left for a fair process other than to allow Ms Rebstock to complete her report and deliver it to the commissioner."
She noted Applicant A had made a statutory declaration that he was not responsible for the leak and that was likely to be noted by Ms Rebstock in her final in the report.
The judges have reserved their judgment and the High Court's suppression of Applicant A's name and employment details will continue at least until they return that judgment.
Mr Rennie has indicated the public servant's name will not be disclosed in any public version of the report.