Cheika has revived the Wallabies in a remarkable 12-month turnaround in which they have gone from a rabble to World Cup contenders.

Their clash against Wales on Saturday night (Sunday morning AEDT) will decide which team finishes at the top of pool A and gets a smoother path through the World Cup play-offs.

Sexton worked with Cheika at Leinster for four years and saw the Wallabies' mentor's angry best. When asked how the Wallabies had turned things around, his response was simple: "Because everyone's probably scared shitless of him."

But Mumm said it was respect, not the fear factor, that was developing the Wallabies back into an international force. "I don't know what Cheik must have done at Leinster," Mumm laughed. "…I have nothing but respect for Cheik and what he's done previously as a coach and also in the limited time I have been with him.

"He's very honest and approachable. I am not afraid of him. Maybe I haven't seen that side, I haven't been through a Dublin winter with him."

The Wallabies have already locked in a quarter-final berth and knocked England out of their home World Cup last weekend. If they beat Wales at Twickenham, they will play either Scotland or Japan in the next stage of the tournament as well as avoiding New Zealand and South Africa until a potential final.

Mumm has been brought back into the starting side to replace Rob Simmons, who has been dropped to the bench for the crucial pool contest. Mumm is enjoying his Wallabies' second-coming after reviving his international career when he returned to Australia in May.

His World Cup has been a whirlwind tour, getting the captaincy for the clash against Uruguay and rushing to London for the birth of his child just days before the game.

The 31-year-old has been playing rugby for the past 12 months without a break. But after a three-season stint with English club Exeter, Mumm says he has a new lease on his international career. "I think I've grown as a player, I certainly have a greater appreciation for what it is to play for the Wallabies," Mumm said. "I think I'm a better, more well-rounded player now than I was."

His first job is ensuring the Wallabies' lineout functions properly as he takes over the calling duties from Simmons. The Wallabies have picked Sean McMahon to replace Michael Hooper at openside flanker, meaning they remain one lineout jumper short.

Mumm is the leading lineout stealer at the World Cup, with four to his name. "I have no doubt [Wales will try to target us], they've picked a very tall lineout and experienced team, maybe that's their way," Mumm said.

"But the focus tends to be on us as a lineout and what we can do. We know we have to improve on that as an element, and the percentage we win per game needs to keep increasing, and with what we can do in terms of mauling, that presents greater opportunities for what we can do in attack."