Ben Kepes isn't happy with the customer care from United after his carry-on bag, containing his passport and wallet, was taken from under his seat.

A Kiwi traveller has been reunited with the carry-on bag that was mistakenly taken off his United Airlines flight more than 20 hours after touching down in San Francisco.

Ben Kepes said he was stranded in the city without a wallet or passport after a United Airlines flight attendant accidentally removed his carry-on bag from under his business class seat and left it in Nashville before departure.

The New Zealander said it was not until his flight reached San Francisco that he realised his bag was gone, leaving him with a dead phone, no charger, no identification and no money.

United spokesperson Jonathan Guerin initially said the bag was inadvertently removed from the plane during a stopover in Chicago, then mistakenly sent back to Nashville, implying it was a checked-in bag in the hold.

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Guerin has since admitted this explanation was a mistake and that Kepes' bag was accidently removed by a flight attendant in Nashville.

Kepes, who lives in Canterbury, went to the US for business and pre-booked a hotel room and prearranged for a car to collect him. He was able to stay overnight in downtown San Francisco and borrow a phone charger.

He expected to fly back to New Zealand on Wednesday night, but was unsure with his delayed passport and essentials.

Hey @united you have my details, I'd expect a detailed update on where my bag, passport and wallet are and suggestions for how you'll remedy — Ben Kepes (@benkepes) May 24, 2017

And I'm supposed to fly international in 12 hours and no visibility over where my bag is or how @united will put it right #avgeek #paxex https://t.co/tyCIkAlvAE — Ben Kepes (@benkepes) May 24, 2017

The United flight attendant thought Kepes' bag was from a previous passenger and was "in tears" when she realised the mistake, he said.

"They didn't ask if it was my bag," Kepes said.

"This was a total screw up, but the individual who made the mistake felt bad and tried to help. It's what happens thereafter that is important."

Clarification: The flight attendant screwed up, but the real issue is how @united deals with the mistake. That's what makes me angry — Ben Kepes (@benkepes) May 24, 2017

That is where he says things went more wrong, describing the customer care he received as "abysmal".

He said baggage services were disinterested and all United Airlines help merely pointed to an online claim form for delayed baggage.

There was no compensation for his delayed essential belongings.

Hey @united - I don't actually want to wait three days before sorting my stuff and making a claim. I need things NOW. — Ben Kepes (@benkepes) May 24, 2017

"[It] was a mistake, and these things happen, but their customer service post that event is what is really abysmal – it's the putting right that counts and they're not doing that to any great degree."

Kepes' belongings were found and United sent a tweet to him, confirming the bag was on a flight en route to San Francisco on Wednesday (US time).

"But [United] didn't even suggest the possibility of getting [my bag] to me – I am expected to go down there to collect it."

"They should have done better I reckon."

FFS - @united IS getting my bag to SFO but... I'm expected to schlep down there to pick it up. Customer service #Fair #PaxEx #avgeek — Ben Kepes (@benkepes) May 24, 2017

The airline has been under fire this year after a passenger was forcefully dragged off a flight so a crew member could have his seat.

Another United Airlines incident saw a woman endure a 28-hour flight after boarding the wrong flight.

In a different flight a scorpion stung a passenger.