"I'm a long time vocal supporter of our brothers and sisters in China that are in detention camps over there and facing disgusting injustices," said Williams, who became a Muslim 10 years ago.
"I just speak my truth - sometimes I get it wrong but in this instance I definitely know I'm not. I'm almost embarrassed to say I don't speak up enough, not just as a Muslim but as a human as well.
"Sometimes you can speak your truth, whether that can get you offside with people or not.
"But I've always been one to stand up, not just for myself, but for what I feel is right - and in that situation, that's all it was."
China does not take criticism from sports stars well. When Arsenal footballer Mesut Ozil tweeted critical comments, they blocked the club's games on TV and deleted the player from video games.
Chinese representatives in New Zealand even fired back at Williams last year after his tweet.
"We're firmly opposed to any attempts to sabotage China's sovereignty and territorial integrity," a spokesperson of the Chinese embassy in New Zealand told Newshub.
"Certain people seemed to be blindfolded by some fake news and clouded by falsehoods. We hope they could view China's counter-terrorism and deradicalization efforts in an objective way."
The 34-year-old Williams - playing on a record $10m deal in the Super League - said a backlash did not worry him.
"You think you know what is hard, what is pressure, what are tough situations," he said.
"But then, take yourself out from where you are. We wake up every day and we can do what we want. But there's people around the world that can't even eat, that have to watch out for bombs being dropped on their heads."
China has detained more than a million Muslims in Xinjiang province in what it describes as "anti-extremist re-education centres".
