"The card holder has not been to Ireland for almost 2 years and has no knowledge of the charge," the official said.
"It was most certainly not authorised by the cardholder or anybody else in this organisation."
The bill has since been reimbursed by the bank.
Mr Key's credit card statements also showed that the Prime Minister took nearly $6000 in gifts on his trip to Turkey, where he attended WWI commemorations, and the Arab States, where he was lobbying for a free trade deal.
The records, which are released each quarter, revealed which ministers and MPs spent the most in the last three months.
The statements showed a big increase in Cabinet-approved travel, which was up $250,000 from the same period last year.
This largely due to a big increase in travel by Trade Minister Tim Groser and Murray McCully, who typically have the largest travel bills.
Mr Groser was the biggest-spending minister, clocking up $197,000 in international travel in the last quarter.
That included trips to France, the Phillippines, Mexico, and the United States for trade negotiations.
Mr McCully was the second-biggest spender. Trips to the Middle East, the United States, Barbados, China and the Pacific Islands led to costs of $172,000.
The biggest spending MPs were National MP for Waitaki Jacqui Dean, with $38,537 in expenses, and Labour MP for New Lynn David Cunliffe, who spent $35,261.