Lolesio received a shock call up on Thursday to play his first game for Australia, and will deputise for James O'Connor in the must-win penultimate Bledisloe Cup Test in Sydney.

Brumbies teammate Irae Simone will also be handed his first cap, starting at inside centre as one of five changes to Dave Rennie's side which was soundly beaten in Auckland a fortnight ago.

McKellar's Super Rugby skipper Allan Alaalatoa comes into tight head prop for Taniela Tupou while Dane Haylett-Petty and young gun Jordan Petaia will also start.

But it's Lolesio's rapid rise to the heralded Wallabies' number 10 jersey that has turned the most heads, with the 20-year-old now tasked with rescuing the Bledisloe Cup campaign. He was part of Rennie's match-day squad for the first Test of the series, but spent 80 minutes sitting on the bench.

McKellar said the former Tuggeranong Viking, who orchestrated last month's Super Rugby AU grand final win over the Queensland Reds, was ready for international level.

"His best qualities are between the ears, he's very confident in his own ability, he's a rare player who has the ability not to dwell on mistakes," McKellar said.

"If he makes a mistake or two he won't let that play on his mind for the rest of the game and then impact on the rest of his performance and others around him.

"There's a good natural footballer who sees space and opportunity, he's a genuine running threat and he's brave, but for me it's his qualities with his mindset and his mental toughness.

"Even in the grand final, he hadn't played in over two months. I knew off the back of his preparation he'd do a good job for us and he went out there and performed well and just did his role really well."

Lolesio was instrumental in last month's Super Rugby decider, just his ninth match for the Brumbies.

Saturday's Wallabies debut will mark a sensational rise to the top for Lolesio who just two years ago helped Tuggeranong to a John I Dent Cup title.

"I remember as an 18 year old when he got thrown into the Vikings first grade side - he got thrown into the grand final and guided them to a premiership, he was one of the best players on the ground," McKellar said.

"He did the same in NRC, he did the same at Super Rugby level and now he's got to do it at international level.

"All he's got to do on Saturday night is just understand his role and...just do the simple things really well, don't think that he's got to go out there and be creative or be the best player on the field.

"When the chances come for him to run the ball, or challenge the opposition defence, he'll see that naturally."

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The Wallabies and All Blacks drew the opening Bledisloe Cup Test 16-16 in Wellington, before New Zealand hit their straps a week later at Eden Park and notched a 27-7 triumph.

Only an Australian win on Saturday will keep the four-match series alive.

Standing next to Lolesio in the backline on Saturday will be a familiar face in Simone, who McKellar said had been rewarded for a superb season at Super Rugby level.

"He's had a really good season for the Brumbies, he's in the best shape he's been in," McKellar said.

"He's always been a big body. At some stages he was too big and just lacked that ability and little bit of sharpness to take space and opportunity.

"He's going to give them really good voice and really good direction and vision. That's what they need with that second playmaker role, he's going to make Noah's job easier and Nic White's job easier. Like Noah, he's primed and ready to go."

WALLABIES: 1. James Slipper 2. Brandon Paenga-Amosa 3. Allan Alaalatoa 4. Lukhan Salakaia-Loto 5. Matt Philip 6. Ned Hanigan 7. Michael Hooper (c) 8. Harry Wilson 9. Nic White 10. Noah Lolesio 11. Marika Koroibete 12. Irae Simone 13. Jordan Petaia 14. Filipo Daugunu 15. Dane Haylett-Petty

Reserves: Jordan Uelese, Scott Sio, Taniela Tupou, Rob Simmons, Fraser McReight, Tate McDermott, Reece Hodge, Hunter Paisami