Hong Kong airline Cathay Pacific has said it will honour first- and business-class tickets mistakenly sold to travellers for the price of economy fares.

First-class fares from Vietnam to the United States, which usually cost about $23,000, were selling for just under $1,500 during a brief period when premium ticket prices plunged.

Business-class seats went for less than $1,000, and there were also cheap premium flights to Canada on offer.

Travel bloggers spotted the cut-price fares on December 31 and shared the information on social media, allowing a lucky few to snap up tickets before Cathay Pacific pulled them from sale.

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Mac Jaehnert, who said he bought one Transpacific flight in February and another in September, tweeted that his New Year's resolution was "to spend as much of 2019 as possible eating caviar and napping in the sky".

As the news spread online, people began speculating Cathay Pacific may not honour the tickets — as doing so would likely prove very costly.

But about 24 hours after the New Year's "surprise", the airline finally addressed the mix-up.

"We made a mistake but we look forward to welcoming you on board with your ticket issued," a tweet to customers said.

"Hope this will make your 2019 'special' too!"

Twitter users who said they had bought the cheap tickets lined up to welcome the news, though at least one questioned whether it was a publicity stunt — something an airline representative denied.

"No airline would ever want that to happen to them," they wrote.

It is not clear how many discounted tickets were sold.

The error wraps up a challenging few months for Cathay Pacific, which revealed in October a massive data breach that affected 9.4 million passengers.

The airline also had to fix the branding on one of its planes in September after it misspelled its own name.