Sonny Bill Williams' generosity has been lauded after he opted to give away his Rugby World Cup winner's medal to a young New Zealand fan. 

The All Blacks beat Australia 34-17 to defend their title at Twickenham on Saturday, with Williams also part of the side that was victorious in 2011 on home soil. 

Amid the post-match celebrations, supporter Charlie Lines was tackled by a steward after trying to get on to the pitch to collect players' signatures, an incident that Williams spotted and acted on. 

"A young fella came sprinting on and he got smoked by the security guard," said Williams. 

"It was pretty sad, he's just a young fella obviously caught up in the moment."

Williams found Charlie in the crowd and handed over his medal, much to the youngster's delight.

"I'm proud and honoured – humbled – to have won so why not try and make a young fella's night," said Williams.

"I'm sure he'll remember it for a while. His mum was pretty happy. He had an All Blacks jersey on so he might be a future All Black."

Williams set up Ma'a Nonu for a decisive try that put the All Blacks out of Australia's reach.

"The coach told me afterwards that the medal was solid gold," Williams laughed.

"I'm pleased that he has it because I know that he'll appreciate it and when he gets older he'll be telling his kids.

"It's probably more special than just having it hanging up on a wall."