The New Zealand Prime Minister was heard whispering an insult about David Seymour, leader of the ACT Party, on a microphone as she sat down after answering his questions in parliament on Tuesday.
'We've always made decisions which we believed to be in the best interests of NZ at the time,' Ms Ardern said, then added: 'Such an arrogant p****.'
Public support for Ms Ardern has tumbled in recent months and her approval rating is at its lowest level since she took office over five years ago.
Ms Ardern lost her temper after Mr Seymour hammered her with a series of questions in the NZ parliament, including one blatantly provocative query.
Mr Seymour asked: 'Can the Prime Minister give an example of her making a mistake, apologising for it properly and fixing it.'
The question prompted a clearly upset response from Ms Ardern shortly before she muttered the insult.
The slur was missed by most members of the house, but picked up in recordings, forcing Ms Ardern into an apology.
She sent Mr Seymour a text message to say sorry.
'I apologise, it's not something I should have said,' the text is understood to have said.
She also added: 'As my mum would say, if you don't have anything nice to say, don't say it.'
Mr Seymour said he accepted the apology and wished Ms Ardern a merry Christmas.
He later used the insult as a springboard to again hammer Ms Ardern in a media interview.
'Some days I am a useless Māori, others days I am an arrogant p****,' he said.
'The apology we are really looking for is for New Zealanders worried about rising prices and ram raids.'
The New Zealand Prime Minister was heard whispering 'such an arrogant p****' about David Seymour, leader of the ACT Party, on a microphone as she sat down after answering his questions on Tuesday
He refused, saying he would need to check Hansard, the official record of proceedings.
Support for Ms Ardern plunged in a recent poll, showing she recorded her lowest approval rating since she took office in 2017.
Labour's popularity fell to 33 per cent while the Nationals rose to 38 per cent.
Mr Seymour has been the leader of the ACT Party since 2014.
The Party won 7.6 per cent of the vote and 10 seats at the 2020 NZ election.
Comment: If Ardern is that easily antagonised, one can only imagine her reaction when under actual pressure when the cameras aren't rolling.
However, questions about Ardern's character have come up on a number of occasions, such as her unusual tendency to grin even during sombre discussions, as well as the seemingly carefree way she talked about New Zealanders being split into a two-tier society over the vaccine mandates: