"This is supposed to be an agency that is created to uphold certain principles as to appropriate ownership."
Mr Hampton said the incident had potential to harm the SFO's court case against Bridgecorp.
"In the hands of a capable questioner, [the incident] can make them look rather embarrassed and lessen their credibility.
"I think it's disappointing. You wouldn't expect, part way through a task, the Commissioner of Police inappropriately celebrating a step along the way in an investigation."
On Saturday, the Weekend Herald revealed an email was sent in Mr Feeley's name on the same day Bridgecorp chief Rod Petricevic was charged over alleged payments of $5.2 million. In the email, Mr Feeley said "it's been a fantastic week" because of the prosecutions against Bridgecorp and other high-profile investigations, including Five Star Finance.
"In light of the Bridgecorp charges being laid, there is a bottle of Gosset champagne [which] needs to leave the confines of my fridge at home and be drunk by those involved with the case," said the email on May 19 last year.
"The relevance of which is that it previously resided in Rod Petricevic's office - and I'll decline to explain how it end [sic] up with me. Hopefully you can all make it to celebrate."
On Friday, Mr Feeley said it was appropriate to acknowledge the effort of his staff who worked on the case.
"I would struggle to think that any reasonable person would consider a $70 bottle of wine an outlandish recognition ... I doubt that any reasonable person would take issue with the use to which the drink was put."
Mr Feeley said he came across "three or four bottles" of champagne in his job as chief executive of the Eden Park Redevelopment Board, based in the former Bridgecorp headquarters.
"One bottle was kept with the expectation that it would be drunk at the conclusion of the project. I ended up at SFO before the completion of the project, and considered that, there being no better claimant to the bottle, it was not unreasonable to recognise the completion of a major investigation and the efforts of the staff with a drink."
Yesterday, New Zealand Law Society president Jonathan Temm said: "It's a $70 bottle of champagne. It's not a legal issue. It's not a story."