Sonny Bill Williams opens up about his love of sport, being a father, faith and criticism he has faced along the way. Video / Māori Television

Sonny Bill Williams has revealed his pride after his wife Alana's decision to adopt the hijab.

The 37-year-old former NRL and rugby great and now professional boxer posted a family photograph to Instagram on Wednesday showing his wife in a head scarf.

"My wife's a hijabi! Alhamdullilah my wife decided to put the head scarf on. One of the proudest moments of my life," Williams wrote.

My wife’s a hijabi! Alhamdullilah my wife decided to put the head scarf on. One of the proudest moments of my life. What makes it so special is knowing she has done it for Allah SWT and no one else. May the most high always bless you with good health, contentment & happiness.❤️🤲🏾 pic.twitter.com/5clPriIpxv

"What makes it so special is knowing she has done it for Allah SWT and no one else. @lanamarieewilli May the most high always bless you with good health, plenty of wisdom, contentment & happiness."

It's a marked change from the wardrobe Alana used to don before meeting the devout Muslim. She worked as a Brazilian nightclub dancer, wearing gold embellished bikinis.

Williams met Alana at a retail shop she was working at in 2013, but she was initially reluctant to give the football star her phone number.

A couple of weeks later, the pair went on a chaperoned date accompanied by her cousin and cousin's husband.

Williams revealed in his autobiography You Can't Stop The Sun From Shining, he and his wife got married just four weeks after meeting, admitting the couple were not in love when they tied the knot.

The dual-code international was renowned as a ladies man in his younger days, claiming he "was a slave to my lusts and desires" before converting to Islam, which helped turn his life around.

"From a Westernised way of living – the way I grew up living – it's a weird concept," Williams told The Saturday Telegraph.

"Straight away we said we're going to do it ­Islamically correct, so even in that first four weeks I wasn't with her by myself, we always had a chaperone."

They now have four children together, and Williams credits their dedication to Islam for ending his days as a womaniser.

"She grew to love my faults, I grew to love her faults, we grew to love each other's strengths," Williams said.

Taraweeh with my girls. ❤️🤲🏾 pic.twitter.com/1Wq3bHV7dM

"I won't lie, one of the biggest tests for me was women. But the discipline I've had and my sporting career, in my life, I've been able to turn that around and strive to be authentic and be a good man, and trying to be the best husband I could be.

"Our shared faith and living for something greater than oneself is what I put it down to.

"Our life is a work in progress, me and my wife. It's not all roses, but we have moments of really good times."

In the book, Williams admits to having a problem with alcohol in his younger days but says people at the Bulldogs were more concerned with "managing a situation", rather than helping him deal properly with his issues.

"The drinking was a symptom of a much bigger issue that was never once addressed and nobody paid any attention to that," Williams writes. "I did need help, but a short course like the one I was signed up for wasn't the medicine I needed for the deep wound I was carrying."

Williams says he's "ashamed of the things I did back then", referring to his introduction to the NRL when he debuted in first grade as a hugely talented teenager. He partied lots, took drugs and of course there was his infamous liaison with former Ironwoman Candice Warner in the bathroom of a Sydney pub, which he speaks about in his book.

"There were other moments when, caught up in the party hard culture after a game, I drank too much, took drugs and let myself down," Williams writes.

"And then there was an incident in a nightclub in 2007, when I fooled around with a woman in a toilet stall.

"Both she and I will have to live with that mistake for the rest of our lives. It made headlines around a good part of the world. It was embarrassing, and not just for myself. There's the woman involved, of course, though she was a single adult woman and so it really was no one else's business what she did.

"But I had a girlfriend at home who was publicly humiliated as well as suffering the pain of being cheated on. She surely did not deserve to be at the centre of a media storm."

As long as he kept performing on the field, the off-field issues could be kept at bay, but Williams knew something had to change because he was "making bad choices".

"I understood the reason I was struggling. The party culture, the money, the access to drugs and alcohol. I was wrestling with it but I wasn't saying no," Williams writes.

"I was selfish and I had no one to answer to. Like so many young people in today's society, I had no boundaries and I was struggling to put some up on my own. It was up to me, but I didn't know what to do about it or how to do that."