A former top adviser to Stephen Harper is "surprised" the prime minister was left in the dark about a secret financial deal between his then-chief of staff and Senator Mike Duffy.

Bruce Carson, who called in to a special edition on the Senate expense scandal on CBC News Network's Power & Politics, said the PM would have been briefed on a matter that significant during his tenure.

He also said sensitive information was always conveyed to the prime minister "immediately" and "no matter how bad it was."

"I guess I could say I'm surprised by it. But I guess each PMO operates differently… but certainly when [former PMO chief of staff] Ian [Brodie] was there we tried to, and certainly I did, tried to make him aware of whatever had to be made aware of, immediately," he told host Evan Solomon.

Harper has insisted he only learned of the $90,000 cheque cut by his former chief of staff, Nigel Wright, to cover Duffy's ineligible expense claims on May 15, through media reports.

Expanding on his comments after the "Powerline" call-in program, Carson said "there would never have been a $90,000 cheque" while former chief of staff Ian Brodie was in in charge.

"But if there had been for whatever reason, the PM would know — could have been Ian or maybe both of us telling him," he said. "I would say that it is surprising that the PM didn't know, because he should have been told. I believe that the PM didn't know — I am just surprised he was not informed."

Not aware of secret fund

Carson, who is facing accusations of influence peddling and has a history of fraud convictions, also said he was not aware of any secret Conservative Party fund that operated out of the prime minister's office. CBC News has revealed that Wright had control and signing authority over the fund.

There is little oversight over the fund, since Elections Canada and the auditor general have no authority to review it.

"I was surprised as anyone to hear about the existence of such a fund, and certainly at the beginning of the [Guy] Giorno PMO, I don't think there was such a fund either, but that's just me not knowing whether there was or there wasn't," Carson told Power & Politics. "But I'd be pretty much certain that there wasn't certainly during the period of time I was there."

Carson said the "drip-drip-drip" of information — often negative — about the Senate expense scandal is likely "debilitating" for PMO employees and Senators trying to get work done.