Rugby convert Sonny Bill Williams has revealed he is eligible to represent the Wallabies because he has an Australian grandmother.

Williams played rugby league for New Zealand and is also eligible for the All Blacks and Samoa.

But his Australian maternal grandmother means he could potentially play for Australia under IRB regulations.

The sticking point is that Australia and New Zealand only pick players from the Super rugby competition, meaning Williams would need to return from French club Toulon to be selected for either country.

"My grandma, she's full-blooded Australian, I think she grew up in Paddington," Williams told the Australian Rugby Union's website, www.rugby.com.au.

Told that meant he could eventually play for any of three countries, he said: "I guess so. I've always just said I'm half Kiwi and half Samoan but I guess I am a bit Aussie.

"When I was young it was funny because I always used to think my Nana was a bit strange, it was just that Aussie sense of humour and when I came over here I knew what she was on about.

"She's a good old lady."

Williams played his first rugby match on Australian soil for the Barbarians in Saturday night's 55-7 loss to the Wallabies at the Sydney Football Stadium.

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