In 2004, when eighteen years old, Williams made his NRL debut for Canterbury-Bankstown against the Parramatta Eels at Telstra Stadium. In 2004, he was selected by New Zealand after only a handful of NRL games and on 23 April made his debut for the Kiwis as their youngest-ever Test player in the 2004 ANZAC Test against Australia. He had previously played for the Junior Kiwis. Williams played fifteen NRL matches during the season, establishing himself in the Canterbury squad. He experienced premiership success in his debut season and became the youngest person to play for Canterbury-Bankstown Bulldogs in a grand final when playing off the bench in the Bulldogs' 16–13 victory over the Sydney Roosters in the 2004 NRL Grand Final. Williams capped a successful debut season by receiving the 2004 RLIF Awards' "International Newcomer of the Year Award" and being named in "Rugby League World" magazine's 2004 World XIII. As 2004 NRL premiers, Canterbury-Bankstown faced Super League IX champions, the Leeds Rhinos, in the 2005 World Club Challenge, which Canterbury lost 32–39.