The Wallabies snatched the last spot in the World Cup semi-finals with their last-minute 35-34 win against Scotland on Sunday (Monday morning AEDT).

They join Argentina, New Zealand and South Africa as the last teams left standing at the tournament, with no northern hemisphere teams making it through to the semi-finals for the first time in World Cup history.

The Wallabies won the Rugby Championships two months ago when they beat New Zealand in Sydney after notching victories against South Africa and Argentina.

But Cheika says that has all been blown out of the park by World Cup form, adding that the Wallabies are the underdogs because he is a self-described international novice.

"What it means [having four Rugby Championship teams left] is that everyone knows each other very well," Cheika said.

"I suppose for us, we're the junior member of the four because I've only been coaching one Rugby Championships.

"[Argentina coach] Daniel [Hourcade] has been there four years, as has Steve Hansen [New Zealand] and Heyneke [Meyer of South Africa] as well.

"They've got more experience than me in preparation of Rugby Championship. We'll prepare the best we possibly can and set out a plan to perform on Sunday."

Cheika has been leaning on his international inexperience for the entire World Cup.

He took over the Wallabies' job 12 months ago when he was thrust into the role after Ewen McKenzie quit.

But while his Test coaching experience is limited, he led Australia to the Rugby Championship title this year, won the Super Rugby championship with the NSW Waratahs last year as well as claiming the Heineken Cup with Leinster in 2009.

New Zealand fired the biggest warning shot of the quarter-finals with their 62-13 demolition job of France while South Africa narrowly escaped against Wales.

The defending champion All Blacks will play South Africa for a spot in the final while the Wallabies battle Argentina in the other semi-final.

Argentina were superb in a 43-20 win against Ireland to seal their spot in the last four and their attacking brilliance will have the Wallabies on edge.

The Wallabies have lost just three of their past 10 Rugby Championship matches. They have lost just one game against Argentina since 1997, but that one win triggered the demise of coach Ewen McKenzie last year.

"Now we're into the semi-finals and we've got to prepare to do our absolute best there against a team we know from the Rugby Championship and a team we know who's a very, very difficult opponent," Cheika said.

The Wallabies struggled to hit top gear against Scotland, making poor decisions and too many mistakes as they snuck through with a Bernard Foley penalty in the 80th minute.

Veteran Wallabies winger Adam Ashley-Cooper said Argentina had improved "out of sight" since joining the Rugby Championships in 2012.

"They're a World Cup contender," Ashley-Cooper said.

"They proved that [against Ireland] with that result and they've proved that throughout the World Cup with their performances..

"Over the next couple of days we just need to address the little areas that need tidying up from the game [against Scotland] and we're really excited about playing Argentina on Sunday in a World Cup semi-final.

"We need to work on a few little things, like exiting out of our own half, relieving pressure a little bit more."

Argentina scored four tries and showed some attacking brilliance against Ireland to race away with an easy victory.

"They're a quality side and have real strike weapons," said Wallabies winger Drew Mitchell.

"Not only strike men, but they have creativity in their halves at 10 and 12. We know them from the Rugby Championship, but at the same time we will have to go away and work hard this week."