A 2012 tweet shows Jacinda Ardern questioned the prospect of Boris Johnson becoming UK Prime Minister, and said he was known as "the gaffe man" when she lived there.
"Are people really discussing Boris Johnson as possible candidate for PM?" Ardern's tweet said. "When I lived in London he was known as the gaffe man!"
A "gaffe" is an unintentional and sometimes embarrassing remark, which Johnson - the current Prime Minister of the United Kingdom - is known for.
During his time as a columnist for the Daily Telegraph, Johnson described gay men as "tank-topped bumboys", and referred to Africans as having "watermelon smiles".
Ardern was a backbench list MP at the time she made the comments. Before being elected to Parliament in 2008, Ardern worked in the UK as a policy advisor to former British Prime Minister Tony Blair.
Ardern's tweet about Johnson didn't go down well with National MP Judith Collins, who re-tweeted it on Monday, saying her words were not "kind, clever or even correct".
The Prime Minister's office has been contacted for comment.
It's not the first 2012 tweet that's come back to haunt Ardern. That year she revealed her true thoughts on Australian radio shock-jock Alan Jones, who in August labelled her a "complete clown".
Ardern had her first face-to-face meeting with Boris Johnson in September, after he took over from former Prime Minister Theresa May who stepped down in July over Brexit deadlock.
Johnson took over as head of the Conservative Party and became Prime Minister. But he was also unable to progress with plans to take the UK out of the European Union because his party did not have a majority in the House of Commons.
Johnson now has the majority he needed, after a snap election was held in the UK last week delivering the Conservative Party their biggest victory since 1987.
Ardern congratulated Johnson in a statement on Friday, saying she had texted him to offer her compliments on the victory, and looks forward to working with him.
"New Zealand and the United Kingdom are close friends and despite our distance, we are strongly connected by our history and people," Ardern said.
"I look forward to continuing to work with Prime Minister Johnson on a wide range of issues as he looks to progress Brexit... I know that Prime Minister Johnson has an affinity with New Zealand having travelled here previously.
National leader Simon Bridges earlier this year described Johnson as having "buffoon-like qualities".
The Opposition leader also issued a statement on Johnson's victory last week, saying he enjoyed discussions with Johnson when he visited New Zealand as Foreign Secretary in 2017.
"I'm sure that once the United Kingdom is in a position to further cement these areas of greater cooperation, including a Free Trade Agreement, New Zealand will be at the front of the queue."