New Zealand's Sonny Bill Williams has gifted his Rugby World Cup winners medal to a young fan after the boy ran on to the pitch at Twickenham during their celebrations.

Steve Hansen's men clinched the Webb Ellis trophy for the second successive tournament courtesy of a 34-17 triumph over Australia in the final.

Williams along with his teammates were celebrating their win with a lap of honour around the ground when Charlie Lines ran on to the pitch to hug the 30-year-old, but he was tackled by security in the process.

The centre brushed the security official aside to embrace the supporter before he walked him back to the sideline where he then presented him with his winners medal.

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

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

"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 being hanging up at home or something like that."

The centre was also part of the All Blacks team to win the 2011 World Cup.