Sean McMahon’s head is still spinning ahead of his Test debut but Wallabies coach Michael Cheika has challenged the rookie to make this the beginning of a long and prosperous international career.

The 20-year-old Melbourne Rebels flanker will start for Australia against Wales at Cardiff’s Millennium Stadium on Saturday (Sunday morning AEDT) after impressing in last weekend’s tour-opening 40-36 win over the Barbarians in London.

It continues a remarkable year for McMahon, who has excelled with the Rebels, National Rugby Championship side Melbourne Rising and Australia’s under 20s.

McMahon also won a Commonwealth Games bronze medal with the Australian sevens but Cheika wants him to see a Test call-up as another huge opportunity, rather than a reward.

“There’s been some chat that it’s a great way to cap off the year and all that business with Sean but it’s actually just the start for him,” Cheika said on Thursday.

“This is his opportunity to really build himself something.”

McMahon was one of Australia’s best players in the 40-36 win over the Barbarians after getting his chance when Scott Higginbotham was forced off with a hamstring injury midway through the first half.

“He wasn’t scared to carry the ball or get involved and to be aggressive, run and tackle hard – the basics of rugby really,” Cheika said.

“It’s not complicated. Sometimes it’s just about taking the opportunity given to you.”

McMahon was hoping his performance may earn him a spot on the bench against Wales but he never expected to be running into one of the world’s great rugby venues as part of the starting side.

“A spot on the bench I would have been more than happy with but this is something pretty special,” McMahon told AAP.

“It’s starting to kick in a little bit. There’s a little bit of nerves but mostly excitement.

“I’m just going to go out there and do my job and use the experience, the learning and the knowledge I’ve gained over these last couple of weeks.”