The High Court has heard it was correct for Ministers to be informed about Winston Peters' pension overpayments, months out from the last election.
The Deputy Prime Minister says his reputation was damaged in a politically-motivated leak of the information.
Sir Maarten Wevers has made a career advising Prime Ministers and officials - but on Monday he was in the High Court as an expert witness.
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When asked whether Ministers should have been alerted to Winston Peters' pension overpayments, Sir Wevers said: "In my opinion, based on my experience, it was appropriate for Mr Boyle to brief Minister Tolley on the no surprises basis here."
Peters is suing over what he has labelled a "political hit job", when details of his benefit overpayments were leaked to media, including Newshub.
Ministers Anne Tolley and Paula Bennett were then notified under the no-surprises policy.
Sir Maarten says the issue was significant enough to qualify.
"The overpayment to Mr Peters and MSD's [Ministry of Social Development's] response to it was a matter that was significant because it could call into question the integrity of the operation of MSD's benefit system," he said.
He added it was even more important given the publicity at the time around Metiria Turei.
The former Green's co-leader admitted benefit fraud - which invoked a public firestorm.
State Services Commissioner Peter Hughes also appeared as a defendant, telling the court that at no point in his career had anyone accused him of political bias.
Hughes went as far as to say that he is confident not even his family knows what his personal politics are.
"The allegations I acted politically are untrue and unfounded and I absolutely reject them," he said.
Former Ministry of Social Development CEO Brendan Boyle also gave evidence on Monday.
He too reaffirmed his decision to tell his then Minister, Paula Bennett.
"This was a significant matter that went to the heart of the integrity of MSD's benefit system which the Minister was accountable for," Boyle told the court.
On Monday, Peters lawyer suggested his name could have remained anonymous.
He is seeking $450,000 in damages.
Newshub.
