A Dunedin Hospital audit of mail delivery times showed some letters took five business days to reach outlying areas.
Under the Official Information Act, the Southern District Health Board released results of an audit in April, which had been prompted by concern over delivery times. Letters to Queenstown were the slowest, taking five business days to turn up.
Winton letters took four business days; Cromwell, three business days; Outram, Waitati, Ocean View, Arrowtown, Clyde and Oamaru, two business days.
In most of Dunedin, letters arrived in one business day, and took two business days to reach Invercargill.
DX Mail has held the contract for the board's mail since 2013, but uses New Zealand Post's network for letters outside of Dunedin, Invercargill and Gore.
Chief executive Carole Heatly said New Zealand Post was investigating the matter.
''Our audit shows that all letters arrived within five working days.
''DX mail was 100% within key performance indicators (one to three working days), and New Zealand Post delivery times to Winton and Queenstown were outside KPIs (three days),'' Ms Heatly said.
In response, New Zealand Post Otago area manager Murray Rei said some of the test letters to Winton and Queenstown appeared to have missed delivery targets ''but we have no evidence of that''.
New Zealand Post requested scans of envelopes from DX to confirm when the letters lodged in its network, but had not received them, Mr Rei said.
Asked if the delivery day reduction would worsen the delays, Mr Rei said the change was designed to ensure 95% of standard mail was still delivered within three working days.
In the first week of July, New Zealand Post mail deliveries will be reduced to three days a week in urban centres.
