While all eyes have been on Jacinda Ardern since she stepped up as Labour leader in August, there's one man who's been quietly advocating for the 37-year-old all along.

Her partner, Clarke Gayford, told Ryan Bridge on The Project he's in awe of Ms Ardern in a candid interview.

"You should be of your partner shouldn't you? I admire her ambition. And I've been watching her hold herself back in some regards... and now she's got a long, long way to go."

Ms Ardern says the campaign isn't wearing her out - but admits Mr Gayford may not feel the same.

"She's speaking for herself, I'm exhausted!" he says.

"When I first got back, I went down to Wellington to catch up and her secretary booked me a 15 minute lunch window. I went up to her new office to see her and we had four other gentlemen sitting with us giving her policy advice, while I ate a sad sandwich going… It's nice to see you."

"Not our most romantic working lunch that we've ever had," Ms Ardern added with a laugh.

But Mr Gayford says it's "absolutely" nice to get a chance to spend some time together.

"I've been riding the highs and lows a lot more, Jacinda's been quite smooth.

"It's been wonderful to watch because I've been overseas filming. And then I keep coming back and watching her go up, and up, and up. It's amazing watching how people react around her.

Mr Gayford says he's learned "the hard way" not to give Ms Ardern policy advice.

"Everyone wants the best for her so everyone's offering her advice so I what I've learned along the way is sometimes it's better just to put my arm around her and give her shoulders a rub."

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