New Zealand are yet to play their best rugby at the World Cup, All Blacks backline ace Sonny Bill Williams warned ahead of Sunday's semifinal against trans-Tasman rivals Australia.

The All Blacks cruised through the pool stage and overcame Argentina 33-10 in the quarterfinal to set up a clash with the Wallabies, who found themselves in this side of the draw after surprisingly losing to Ireland in their pool before overcoming defending champions South Africa in a tight quarterfinal.

"We haven't played our best footy yet," said Williams, who spent five years in Australia playing Rugby League including winning the NRL grand final in 2004 at the age of 18.

Williams, who also won seven New Zealand league caps before switching codes, has found himself playing in the unaccustomed position of wing as the All Blacks battle several injuries within their squad, but the 26-year-old insisted he was enjoying the experience.

"I've surprised myself how well I've played on the wing," the league second-rower, whose smart offloads and running lines have been among the highlights of this tournament, said on Tuesday.

"Some people didn't think I could play rugby (union), I showed them that I can. I'm always a confident player and if I'm put out there (on the wing) I'll do the job as best I can, just do the little things and it all comes good.

"I'm doing all the little things right, I'm ticking all the boxes during the week and staying in the now, not looking back at the past or what's happened," he added.

The World Cup, Williams argued, was the reason he moved to French Top 14 club Toulon in 2008 and then back to New Zealand last year.

"There is nothing better than this stage," he said. "It's pretty much the whole reason why I came back here, to test myself against the best players, to try and make this team, this squad."

Now that fullback Mils Muliaina has been ruled out with injury, Israel Dagg will be favourite to take over the No.15 jersey against Australia, leaving coach Graham Henry with a real selection headache when it comes to Williams.

Henry rightly likes the proven midfield combination of Ma'a Nonu and Conrad Smith, likely leaving Williams to battle Richard Kahui and Cory Jane for a spot on the wing.

Alternatively, the All Blacks might think that pitching Williams in at wing against proven Wallaby finishers Digby Ioane and James O'Connor might be too much to ask, leaving the cross-code star kicking his heels on the bench.

"Competition is fierce within the team," Williams acknowledged. "But it's a good competition and it brings the best out of us. Hopefully, I'll get picked and I'm involved. If I am, that's great."

AFP