Australia boss Michael Cheika had few qualms branding his Wallabies the underdogs for Saturday's World Cup final against defending champions New Zealand.

Head coach Cheika insisted Australia must produce "something extra just to be competitive" against the All Blacks, with both sides bidding to lift the Webb Ellis Cup for a record third time at Twickenham.

Adam Ashley-Cooper's hat-trick and Rob Simmons' early sucker-punch score guided Australia past Argentina 29-15 in London on Sunday, teeing up that trans-Tasman showdown with the All Blacks.

Cheika would not go as far as to agree with Springboks boss Heyneke Meyer hailing Steve Hansen's New Zealand the best side of all time, but still accepted the trophy holders will start as favourites.

"Well you've got to go back into the history annals to talk on that," said Cheika, when asked if the All Blacks are the best of all time.

"We've got to improve massively from what we did today, to even be in the hunt next week"

"They are obviously the world's number one.

"We've only beaten them once in the last 10 matches.

"So they'll be feeling they've got our measure and it's going to be up to us to do something special, something extra just to be competitive. So we'll see how we go.

"We've got to improve massively from what we did today, to even be in the hunt next week.

"We'll pick two or three things and try to make those things better.

"When you've got a team like New Zealand with so many threats, who are so well-drilled and have such a great coach you have to be working hard all the time.

"New Zealand have had a very different path from us to here. They will be feeling a little bit differently in their preparation.

"But it's a World Cup final and the adrenaline's going to be pumping and anyone who's a little bit sore will forget all about soreness next week."

Former Leinster boss Cheika only assumed the Wallabies job in October 2014, after Ewen McKenzie's resignation. The taskmaster coach has transformed a talented but disparate group besieged by infighting and recrimination into potential world champions, and all inside 12 months.

The no-nonsense 48-year-old still batted away any praise when quizzed on how he has reversed Australia's fortunes however.

"I'm not taking credit for any type of that stuff," said Cheika.

"I don't know if we're turned around or anything, I just know guys are playing for each other, want to play for Australia and are committed when they take the field.

"That's the basis for a contact sport, and that's about it really."