Speaking at an event looking ahead to the tournament, McCaw, a two-time World Cup-winner, said he expects Michael Cheika's team to come good at the tournament, despite having won just four of their 13 games last year.
"Knowing Australians like I do, they are competitive, hugely competitive and they will get themselves in good situation to compete," McCaw, who led the All Blacks to victory over the Wallabies in the 2015 final, told Reuters.
"They have got ability to win games now against everyone so you don't want to write them off at all.
"I think sometimes where people are not talking about them a lot leading up to (the World Cup), that's the best thing you can be. You just come in and surprise everyone.
"But from a New Zealand point of view, there is never a surprise that they front up.
"It is their competitive nature or something; they always come and show up."
Australia have been drawn in a potentially tricky Pool D with Six Nations champions Wales and the vastly improved Fiji in Japan and will enter the tournament without the country's three-time player of the year Israel Folau.
The 30-year-old was sacked after being found guilty of a high-level breach of Rugby Australia's code of conduct after he posted on social media that hell awaits drunks, homosexuals, adulterers, among other groups.
Folau has appealed the decision to the Australian courts.
McCaw refused to comment directly on the case but said it was "sad for rugby".
"Without getting into the debate around who is right or wrong, I just think it is sad for Australian rugby that you are not talking about what Wallabies are going to do," he said.
"But hopefully once the Wallabies get playing, and they are getting to the Rugby Championship and the World Cup, talking around what they are doing from rugby point of view then."
New Zealand begin their quest for a third consecutive Rugby World Cup against old rivals South Africa in a titanic opening Pool B clash in Yokohama on Sept. 21.
It is a game that McCaw, along with the rest of the rugby-world, is eagerly anticipating with the loser likely to face Joe Schmidt's Ireland side in the quarter-finals.
"That is a huge game because both could win and they have to play each other first up," said McCaw.
"It doesn't mean that the team that doesn't win gets eliminated but it certainly will go off in a different direction with different things to think about.
"So, I think it will be a big game and it will show where everyone is at."
Photo Wallabies coach, Michael Cheika