The man worked for five years alongside David Sawtell (32), who was jailed for two years three months on Tuesday before the Dunedin District Court.
He admitted spending a decade trawling the internet for child sex abuse videos and images and police found more than 1000 objectionable files in a cloud-based account - some involving children as young as 2 and featuring animals.
Sawtell's former colleague, whose children are still involved in the movement, spoke to the Otago Daily Times on condition of anonymity.
He never suspected the defendant was leading a secret double life.
''I honestly thought he was just socially awkward. I feel quite naive and quite stupid now,'' he said.
''Scouting worldwide has worked extremely hard in the last few decades to make things safe. It breaks my heart. This has dragged everybody back 10 years.''
He remembered undertaking a two-day course on child protection with Sawtell.
''That just blows your mind,'' the former colleague said, thinking back.
There was also something of an online backlash from parents of Scouts and ex-Scouts who had come into contact with Sawtell during his tenure of more than a decade.
A Mosgiel woman whose son spent three years under the defendant's tutelage said she and others should have been warned.
''It was a bloody shock,'' she said.
''Scouting should have stepped up ... I'm a bit p...ed at them at the moment.''
Scouts regional development manager Chris Wilson said because of court-imposed suppression orders and the ongoing police investigation, they were unable to inform parents about the specifics of Sawtell's departure.
Leaders were instructed to tell parents he had ''been exited'' without explaining the circumstances.
Mr Wilson met parents of Sawtell's former group on Monday and explained to them the kind of publicity that was likely to be published in the media yesterday.
He stressed that all leaders were police-checked every three years and Sawtell's clean criminal record would never have thrown up red flags.
Detective Senior Sergeant Kallum Croudis said police had worked closely with Scouts throughout the investigation and advised them on the handling of members' concerns.
''A referral process was put in place, to ensure that the organisation and parents could easily alert police of any reports of concern,'' he said.
''To date, police have received no reports of concern from any parents of current or former Scouts.''
Sawtell is listed as a Swimming Otago Service Award recipient in 2012 and was noted online as a marshal for various meets.
Sawtell's parents were convinced he would have never abused a position of authority and offended against a child in his care.
''At no stage has it ever been suggested that he physically molested kids. It's only at home looking at porn. He's been addicted to it,'' his father said.
''As soon as we found out we went looking for help and you just can't get it.''
Sawtell will appear before the Parole Board for the first time in nine months.
