Deputy Head of Mission Caroline Beresford later deleted the anti-President Donald Trump tweets and made her account private.
New Zealand's Ministry of Foreign Affairs and Trade said the tweets didn't meet its code of conduct for social media, which require diplomatic staff to maintain political neutrality and take care in expressing personal opinions.
"The ministry does not in any way endorse the content or tone of the tweets," it said in a statement. It said it was taking appropriate action, but declined further comment.
Beresford sent at least three tweets late last week disagreeing with an opinion column on political news website "The Hill" that said a 2020 Democratic ticket with Bernie Sanders and Elizabeth Warren could "win big."
"No it couldn't," Beresford responded. "Please get your (expletive) together or we will all die."
She also wrote that Democrats had "learned nothing" and that "If this is what they have against Trump we should be planning for the eight years."
Beresford could not immediately be reached for comment by The Associated Press. In an email to the news website Newsroom, which first reported the story, Beresford confirmed that she had personally sent the tweets, but "realized very quickly that they were inappropriate, which is why I deleted them."
Beresford is responsible for running much of New Zealand's day-to-day diplomatic mission in the United States, and is ranked below only Ambassador Tim Groser.
The comments were jarring in New Zealand, where politicians and diplomats have been careful to avoid making strong public statements about Trump or his policies. New Zealand considers the U.S. a close ally, and the two nations are part of the "Five Eyes" intelligence gathering network, which also includes Australia, Canada and Britain.
New Zealand Foreign Minister Winston Peters said he was aware of the matter and was confident the ministry was handling it appropriately.
The U.S. Embassy in New Zealand said in a statement that it worked closely with its "hardworking, professional New Zealand counterparts" and held them in the highest regard.
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