The head of New Zealand’s public radio station has apologised for publishing “pro-Kremlin garbage” on its website after more than a dozen wire stories on the Ukraine war were found to have been altered.

Most of the stories, which date back more than a year, were written by the Reuters news agency and were changed at Radio New Zealand (RNZ) to include Russian propaganda.

A digital journalist from RNZ has been placed on leave pending the result of an employment investigation.

Paul Thompson, the chief executive of taxpayer-funded RNZ, said it had found issues in 16 stories and was republishing them on its website with corrections and editor’s notes.

He said he was commissioning an external review of the organisation’s editing processes.

“It is so disappointing. I’m gutted. It’s painful. It’s shocking,” Mr Thompson said on RNZ’s Nine to Noon show.

“We have to get to the bottom of how it happened.”

Mr Thompson said the station had forensically reviewed about 250 stories since first being alerted to the issue on Friday and would be reviewing thousands more.

Some of the changes were just a few words and would have been hard to spot by casual readers.

Changes included the addition of pro-Kremlin narratives such as “Russia annexed Crimea after a referendum”, and that “neo-Nazis had created a threat” to Russia’s borders.

The referendum, which was held after Russia seized control of Crimea, was considered a sham and was not recognised internationally.

Russia for years has also tried to link Ukraine to Nazism, particularly those who have led the government in Kyiv since a pro-Russian leadership was toppled in 2014.

Ukrainian President Volodymyr Zelensky, who is Jewish, angrily dismisses those claims.

Former New Zealand prime minister Helen Clark tweeted that she expected better from the public broadcaster.

“Extraordinary that there is so little editorial oversight at Radio New Zealand that someone employed by/contracted to them was able to rewrite online content to reflect pro-Russia stance without senior staff noticing,” she wrote. “Accountability?”

Mr Thompson told the Nine to Noon programme that typically only one person at RNZ had been required to edit wire service stories because those stories had already been subject to robust editing.

But he said RNZ is now adding another layer of editing to such stories.

He said he wanted to apologise to listeners, readers, staff and the Ukrainian community.

“It’s so disappointing that this pro-Kremlin garbage has ended up in our stories,” Mr Thompson told Nine to Noon. “It’s inexcusable.”

RNZ began as a radio broadcaster but these days is a multimedia organisation, and its website ranks among the nation’s most viewed news sites.

Reuters did not immediately respond to a request for comment.