Several of Australia's World Cup football squad on Wednesday are under fitness clouds in the lead up to the tournament in June.

Concerns surround the fitness of several key players, most notably Harry Kewell, although goalkeeper Mark Schwarzer and midfield star Tim Cahill are in perfect health and peak form.

It is arguable Schwarzer and Cahill are Australia's two most indispensable players in their bid to emerge from a difficult Group D in South Africa.

"It's always a concern when guys are injured heading into a big tournament," Schwarzer said.

"But we've still got three weeks or so until the beginning of the World Cup, so it still gives players the opportunity to find their fitness, to overcome any niggles or injuries they've had over the season."

Cahill believes Australia's work ethic and experience from 2006 will be the keys to their performance in South Africa next month.

"First of all we have to keep players fit and then try to get the consistency," Cahill said.

"If we prepare right and try and be as fit as we can for this World Cup like we were for the last World Cup, our natural talents will hopefully shine through.

"We've got great character, great work ethic within the team and a team that sticks together," he said.