A spokesman for United Airlines said the airline "deeply apologises" for the mistake.
"The luggage of a customer travelling from Nashville to San Francisco was mistakenly shipped to the wrong city.
"We located the customer's bag and rerouted it to San Francisco, where it arrived today at approximately 11.42am local time. We are in contact with our customer to apologise for our mistake and to co-ordinate the retrieval of his luggage."
Kepes appeared less than impressed with the airline's customer service, bemoaning the airline's "sheer ineptitude" on Twitter.
Kepes said yesterday he only realised about four hours into the flight to San Francisco that the bag was missing.
"It was only about four hours later, as we were about to land in San Francisco that I realised my bag wasn't there and the penny dropped.
"They didn't ask if it was my bag, or look at the external Air New Zealand tag which has my name and frequent flier number on it."
Kepes was later told by United a flight attendant had seen the bag and assumed it was left from a previous flight.
Kepes said United told him the incident was a mistake and the flight attendant was in tears when she realised what she had done. However, it was the follow-up customer service that annoyed him.
"I went to Baggage Services who were pretty disinterested," he said, "but finally got through to Nashville where I'd come from. The bag, wallet, passport, etc was there."
The incident is the latest in a string of public relations disasters for the airline.
In April, security staff were filmed dragging a doctor off a plane, while another flight the following month was delayed when a scorpion was spotted on board.