New Zealand Post did not act earlier on customer complaints of missing mail in Queenstown's Fernhill area because they had not reached an alarming level.
From January 2010 the company had received 101 complaints from customers not receiving mail, but numbers were still at an "organic background level" up to August this year, media communications manager Michael Tull said.
A 32-year-old Queenstown woman, contracted by NZ Post, was arrested on Friday by police after they were notified by the company of concerns over undelivered mail to the Fernhill area.
She will appear at the Queenstown District Court on Monday on one charge of theft by a person in a special relationship.
Mr Tull said while there had been two complaints a month on average this year, it was not until August when NZ Post noticed a "spike" in numbers and launched an investigation by its national security team.
Before this, the company had launched an internal investigation with the Queenstown branch, which did not "unearth any apparent causes".
Mr Tull accepted the public would raise questions over the complaints-handling procedure and the time it had taken to reach an outcome.
An "experienced postie" would deliver mail to Fernhill this week and four staff members would be sorting through undelivered mail with police and returning "undisturbed" envelopes and parcels to their rightful owners early next week, with apologies.
All other mail would be used by police as court evidence. Mr Tull encouraged any customers who felt they had been disadvantaged by late mail to call NZ post on 0800 501-501.