Why Jacinda Ardern refuses to sing Christmas songs on the radio

Jacinda Ardern has declined to sing a popular Christmas song on a radio breakfast show, saying she couldn't possibly top John Key's infamous effort.

The Prime Minister was on The Edge's Jay-Jay, Dom & Randall show on Friday morning when they challenged her to sing Mariah Carey's festive hit 'All I Want For Christmas Is You'.

She immediately refused, not confident she could hit the diva's high notes.

"Anyone other than Mariah," Ms Ardern told the hosts.

They told her former Mr Key was always game, and played a clip of his wildly off-key warbling from 2015.

"No one can top that. I can't. No one can top that," Ms Ardern said.

"I need to let that stand for Prime Ministers for decades to come. I wouldn't want to ruin his record as the last one to have done it."

Ms Ardern explained she's not opposed to singing in public, but would rather belt out "Smashing Pumpkins and Pantera" than Mariah.

The pros and cons of being the boss

Ms Ardern also spoke about what it's like being Prime Minister. Some aspects, she expected - such as the heavy reading workload.

"That bit didn't surprise me because I used to work with Helen Clark - I used to see the reading, so I went in eyes wide open. It surprises me how long it takes me to read it all."

Asked if she'd considered trying to fit it in while exercising, Ms Ardern said she heard Barack Obama tried that and fell off the treadmill "a couple of times".

Something she's not used to yet is having a security detail.

"There's actually some things that make me feel a bit awkward and uncomfortable - constantly having to move with a pack of guys. People now know where I am because they see the earpieces. I'm not quite used to that. I know the family's going to struggle with that when we go to Christmas."

Jacinda Ardern.
Jacinda Ardern. Photo credit: The Edge/MediaWorks

But she won't tell them to bugger off in case "a couple of dodgy aunts and cousins" decide to try something when the extended family gathers over the break.

But despite heading to Gisborne, 37-year-old Ms Ardern says she won't be attending Rhythm and Vines this year.

"I felt old even when I went two years ago."