Ireland coach Joe Schmidt explains decision to drop Johnny Sexton

Sexton, capped 73 times, was a key component in Ireland's Grand Slam success earlier this year, finishing the NatWest 6 Nations as the tournament's third-highest points scorer with 44.

But Munster-bound Carbery will start at Brisbane's Suncorp Stadium on Saturday with Sexton, who last month led Leinster to European Champions Cup glory, having to make do with a place on the bench.

"Some of those players have had some reasonably tough weeks and it was an opportunity for some of those guys," Schmidt said at his pre-match press conference.

"We really wanted to get Joey out there at some stage and we thought the best way was to give him the longest preparation window.

"It was a surprise to see Joey going to Munster, we didn't know he would get that opportunity.

"But we had already spoken about this and we decided to forge ahead.

"Joey has had three training opportunities to slot in there, as it did for John Ryan and Rob Herring in a similar vein.

"It's a good opportunity for some of those people to put their hands up."

Carbery last week announced that he would be leaving Leinster to join provincial rivals Munster and has only started two of his previous 10 caps, against the United States and Fiji.

In all, Ireland have made six changes to the side which beat England in March to seal Grand Slam success.

Robbie Henshaw comes in for centre Garry Ringrose in the only other change to the backline, while there is a new front row of Ryan, Herring and Jack McGrath.

Jordi Murphy joins the back row from where Peter O'Mahony will lead the side in the absence of injured hooker Rory Best.

Ireland have won three of their last four Tests against Australia, but lost 22-15 on their last visit to Brisbane in 2010.

"They've got a huge amount of caps mixed in with a few newbies," Schmidt said.

"Not too many teams in world rugby would be able to leave out the likes of (Tevita) Kuridrani and not select Reece Hodge and still have an incredibly strong-looking backline."

Forwards Brandon Paenga-Amosa and rugby league convert Caleb Timu will both make their Australia debuts.

The Wallabies have named an otherwise settled starting XV that boasts 623 Test caps between them and includes play-making stars Will Genia, Bernard Foley and Kurtley Beale.