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The inquiry started when Detective Sergeant Dan Keno was given a file by the Online Child Exploitation Across New Zealand (Oceanz) team. The file had come from overseas.
It concerned an Oamaru resident who had posted pictures of his child on an online forum, but ''there was nothing objectionable about those images'', Det Sgt Keno told the Otago Daily Times yesterday.
With no evidence to support a search warrant, Det Sgt Keno visited the man to build up a rapport with him.
''You have to build that rapport, whether it is a victim or an offender, to get the best outcome.''
The man allowed Det Sgt Keno to look at his computer and emails.
''To get a search warrant, we obviously need good cause to suspect that an offence has happened and at that stage we didn't have that ... it wasn't until we went through his emails that it became apparent the extent of what he had done.''
That search uncovered objectionable images, videos, and comments from the man, who had been in contact with people in New Zealand and overseas, including Aaron John Ellmers, who had been offered the Oamaru man's son for $500.
''This is what kicked the whole thing off.''
Ellmers (41) pleaded guilty to 60 charges, including sexual violation, and was sentenced in the High Court at Napier last month to preventive detention, for a minimum period of 20 years.
On Wednesday, a 27-year-old South Canterbury man, who formerly lived in Oamaru, was sentenced to eight years and 10 months' jail for offending against his 13-month-old son.
The man, who had filmed and photographed acts of sexual violation for trade, was given final name suppression by Judge Joanna Maze to protect his son.
Det Sgt Keno, who spent a little more than two years working with a North Island-based Child Protection Team before returning to Oamaru in November 2011, said the arrest ''was a pretty good result''.
The child was in a safe and supported environment at an undisclosed Otago location and was being monitored by Child, Youth and Family, which was also supporting the boy's mother and the extended family.
The family were understandably upset by the experience and were receiving support, he said.
The incident highlighted the importance of internet safety for parents and guardians, he said.
''It opens a lot of people's eyes how accessible these things are with the internet these days. Once you press that button, you don't know where it is going or who it is going to. It's a big, wide world out there.
''Those images have been distributed quite extensively by the offender and that tends to be the case with child exploitation material - it is shared among the paedophile community, both here and overseas,'' he said.
The case had been a major talking point in Oamaru and people were shocked that such a case could occur in the town, he said.
''But it does strengthen those ties with decent people who can't believe that sort of thing goes on in a community.''
How does a case like this affect those police officers involved?''It does impact on you, especially when you have kids of your own. But it makes you more determined to do it properly and see it through.''