Timana Tahu is poised to make his starting debut for the Wallabies by replacing the injured Berrick Barnes at inside-centre for the Tri Nations Test against the Springboks in Johannesburg on Saturday.

Barnes is expected to miss the final two Tri Nations matches, including the Brisbane finale against the All Blacks on September 13, giving the former Kangaroos league representative the chance of making the No.12 spot his own. Wallabies coach Robbie Deans has to make the choice of either playing two midfielders in their preferred positions, which would mean Stirling Mortlock remaining at outside-centre, or moving the Wallabies captain into No.12, and selecting Ryan Cross at outside-centre. The more logical step would be the Tahu-Mortlock combination, especially as Deans has been a major supporter of the convert in his first year of rugby.

After he struggled to get a regular spot with the Waratahs, where he was considered an outside-back, Tahu's year blossomed when Deans took an interest in him at the end of the Super 14 and made sure Australia A used him at No.12. Several good performances in the Pacific Nations Cup had Tahu fast-tracked into the Tri Nations squad.

It has also been clear at several Wallabies training sessions that Deans has kept an eye on Tahu, spending a lot of time at the end of the practice sessions working on his skills and understanding of the game. The poise is already there, as Tahu proved when he was used off the bench in Durban last Saturday and was prominent in the final 20 minutes of the 27-15 victory.

When asked about his centre options, Deans said yesterday: "I'll have a good look at the footage, and it was great for Timana to get the time that he got. We delayed his entry obviously and referred to blokes who had a background in Test rugby first. But when Timana got into the game, he did well."

When asked if his time off the bench on Saturday was enough to convince him that Tahu could now start a Test, Deans replied: "There's no doubt he can start a Test. He's perfectly capable of that, as are all of them."

Fellow Waratah Dean Mumm has also done more than enough to find himself in contention for Dan Vickerman's second-row spot at Ellis Park. Like Barnes, Vickerman has a serious shoulder injury, and is also set to miss the rest of the Tri Nations.

Mumm fitted in perfectly, replacing Vickerman just before half-time, and had a major impact on Australia nullifying one of South Africa's strengths - its line-out.

Mumm's effort was even more incredible considering that six hours before kick-off he was lazing by the team hotel pool in Durban, believing he was going to be a spectator. That is until a Wallabies team official rushed up to him and said that he had better check in with Deans, because the original forward reserve, Hugh McMeniman, had been sidelined with a virus. A quick haircut by Adam Freier, and Mumm was ready.

"I was reasonably relaxed and had no reason to be uptight," Mumm said yesterday. "Before I knew it, I was out there, singing the anthem. On that front, I was a lot more relaxed than what I normally would have been."

Now they have to confront the altitude factor and the fact that the Wallabies have not won in Johannesburg since 1963, which is also the year an Australian team last won two Tests in a row on South African soil. But Deans is not getting hung up by that. When asked what the altitude meant to him, he said: "You breathe a bit harder a bit earlier. But you breathe harder in every game. I wasn't part of the previous [Wallabies] groups that have played there. And nor will I be seeking any insight, to be honest.

"These guys showed yesterday that they are capable. But we would really like to show we are capable of getting up again. To undulate is not good enough and essentially that's what we've been doing up to date. We would like to address that this week."