'You can't beat this moment': Aussie boxing champion Jeff Horn has welcomed a baby girl.

Six months ago, Jeff Horn, a self-confessed “shy nerd” from Brisbane who married his high school sweetheart made headlines all over the world when he defeated the (now former) world welterweight title holder Manny Pacquiao.

But even in the moments after the unexpected win – for which he pocketed a rumoured $1 million – the 29-year-old knew the “best moment of his life” was still to come.

He wasn’t talking about future bouts in the ring. Instead, he was talking about the impending arrival of his first child.

After an 18-hour labour, Isabelle Kate Horn was born at 7:40pm on Saturday, December 30, weighing a healthy 3.58 kg, according to The Courier Mail.

Jeff shared the adorable first snaps with his fans on his Facebook page, adding that "she was the last but best thing to happen to Joanna and I in 2017."

"We can't wait to share the rest of our lives with you."

It's a sentiment the boxer also shared with The Courier Mail in his first interview as a new dad, saying that winning a world title, successfully defending it and being awarded a number of accolades, paled in comparison to Isabelle's arrival.

"Nothing really compares to that, you can't beat this moment," he said.

"She's gorgeous. I had tears in my eyes and even just before talking about it makes me start tearing up."

LISTEN: Monique Bowley and Rebecca Judd speak to a series of dads, about their experience in (or outside) the birthing suite. Post continues after.

Joanna also shared a picture of Isabelle on social media, writing that she and Jeff loved their "absolutely perfect baby daughter" "unequivocally and unconditionally".

Welcome my absolutely perfect baby daughter, Isabelle Kate Horn. We already love you unequivocally and unconditionally. #babygirl #cutebabies #adorable #babies #parents

A post shared by Joanna Horn (@jojohorn87) on Jan 1, 2018 at 2:58am PST

She told The Courier Mail that Jeff was a "natural" when it came to being a new dad.

"I'm really surprised what a natural he is with her," she said.

"I always thought I'd be the more hands on one with her but he's been so wonderful and he might even teach me a thing or two.

"She's just perfect and we can't stop staring at her and thinking oh my gosh we created this."

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