New Zealand's Sonny Bill Williams gave his Rugby World Cup winners medal to a young boy who had been roughly tackled by a security guard as he tried to get on the pitch.

Two-time world cup winner Williams said he had felt sorry for the youngster - who the New Zealand Herald say is called Charlie Line.

"A young fella [sic] snuck on the field somehow but when he was coming to give me a hug, he got smoked by a security guard, full-on tackled him," Williams explained.

"He was only eight and the other fella was a full-on man, so it looked like he would break his ribs or something."

ITV News understands that Charlie may be around 14-years-old - slightly older than Sonny Bill Williams thought.

"The moment probably just got the better of him but he was just so excited to get on the field with the All Blacks.

I thought I'd make it a night to remember for him, rather than my medal hanging up at home or something like that," said the All Blacks star.

"It will be hanging around that guy's neck and he can tell that story for years to come. He might be a future All Black!"

I'm pleased that he has it because I know that he'll appreciate it. When he gets older he'll be telling his kids things like that. That's more special than hanging (it) up on my wall at home.

Sonny Bill Williams

New Zealand defeated Australia 34 - 17 to become the first side to win back-to-back world cups.

Prior to last week's semi-final against South Africa, Sonny Bill took to Twitter to offer two tickets to Syrian refugees.