Courier Post promises overnight delivery - although some residents say it doesn't apply in Carterton.

Book an overnight courier delivery to Carterton and you might find yourself waiting up to four days.

Locals say problems with the Wairarapa town's overnight Courier Post service spiked in late January.

Carterton resident Craig Hood said he had seen Courier Post's once-excellent service take a considerable downturn.

"We quite regularly noticed that we weren't getting things the next day. Not only the next day, we weren't even getting them the day after," he said.

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Though a New Zealand Post representative assured him the problem would be solved several weeks ago, that hadn't proved to be the case.

"It's been going on for several months. I've been told it was a problem with a worker, but it really seems like a management problem. You don't let this sort of thing carry on for several months, you sort it out.

"Sometimes it feels like Courier Post delivers overnight everywhere in New Zealand, except Carterton."

He added the slow delivery could prove particularly troublesome for businesses and, especially, people relying on medical supplies.

New Zealand Post admitted there were issues with its Courier Post service in the town. A New Zealand Post spokeswoman said the company had received a heightened number of customer complaints in late January.

While she attributed some of the delays to a combination of service delivery, public holidays and extreme weather events, New Zealand Post confirmed a new delivery driver would be starting this week.

"As soon as we became aware of customers experiencing delays at the end of January, we began an immediate investigation," the spokeswoman said.

"NZ Post considers this to be a serious matter and what we can say is that all parcels have now been delivered.

"A new courier driver will be starting this week and we will continue to monitor deliveries closely."