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Ms Bennett used the term during a raucous exchange in Parliament about National's Limited Services Volunteer bootcamps.
Ms Ardern had asked Ms Bennett about the drop-out rate - and then interjected by repeatedly shouting "50 per cent" during Ms Bennett's reply.
Ms Bennett suggested Ms Ardern "listen to the answer" before drawing her finger across her mouth and saying: "Zip it, sweetie. I'm getting there."
It prompted roars of laughter from National MPs, but Labour's Trevor Mallard was quick to object, telling Speaker Lockwood Smith it was "exceptionally offensive."
"If that term was used to an MP who was not a younger female member in that sort of approach, I think you would find it offensive."
However, Dr Smith said Ms Bennett's response was "less than ideal" but she had been provoked by Ms Ardern interjecting.
"Nobody can say that's outrageously offensive ... If we allow ourselves to get worked up over that we're just being unnecessarily petty. The solution is simple: don't interject so much."
Ms Ardern was quick to laugh it off on Twitter, saying it was a first for her.
"Kids sitting in Gallery could be forgiven for thinking they were watching a Hairspray revival."
However, Mr Mallard got revenge by calling the Speaker "sweetie" at every possible opportunity.
Other MPs took to Twitter to give their views on the 'zip it, sweetie' comment. Green MP Holly Walker said it demonstrated "what an inherently patronising place it is to be a young woman MP."
She also wondered if the Speaker would have reacted differently if a male MP had said it.
Afterwards, Ms Ardern said she did not feel patronised, but did think it was sad.
"Maybe I just make them angry."
It is not the first time other Labour MPs have come to the defence of Ms Ardern for sledging. National MP Maggie Barry called out "how many children do you have?" to Ms Ardern - who has no children - during a debate about paid parental leave.
- By Claire Trevett of the NZ Herald