Wallabies skipper Stirling Mortlock will shift into second-five eighths if Berrick Barnes is unfit to face the All Blacks with Wallabies coach Robbie Deans certain to avoid risking league convert Timana Tahu as a midfield starter for the crucial Tri Nations decider in Brisbane.

Tahu's defensive inexperience was exposed by the Springboks as they ran in eight tries in last week's 53-8 flogging of the Wallabies in Johannesburg.

With Barnes only rated 50-50 at best to come back from his shoulder injury to front the All Blacks, Deans has a midfield problem he'd rather be without for a match of this significance.

His gamble to introduce Tahu into the hurly-burly of the backline at such an early stage of his rugby career backfired in South Africa.

Now it seems his only option if Barnes is ruled out will be to move Mortlock into the No. 12 jersey and play another league convert, Ryan Cross, at centre.

Cross, who played rugby in his school years, has looked increasingly comfortable in the test surroundings this year after settling in well with the Western Force.

But Barnes' absence will be felt in several areas, not least field kicking where he has been an invaluable second option to first five Matt Giteau.

That would bring the Wallabies back into line with the All Blacks where so much of the kicking responsibilities lie with Daniel Carter at No 10.

Barnes' chances of making the start at Ballymore will be known better when the Wallabies assemble in Brisbane on Sunday, the same day the All Blacks arrive in Queensland.

But regardless of Barnes' health, it seems Deans can't afford to risk Tahu in the staring jersey again.

Former Wallaby centre Anthony Herbert told Brisbane's Courier Mail newspaper that Tahu looked "lost and confused" in defence against the Boks.

"He made a bad read in defence first-up and there seemed a lack of talk to organise the defence, in general, from the backs," Herbert said.

"If Berrick doesn't come right, you've got no choice but to go with Mortlock and Cross.

"I was all for the Tahu experiment after his form with Australia A and he will get better, but you're not going to take the risk on him in a Tri Nations decider."

As for the man himself, Tahu had this to say after his nightmare in Johannesburg: "I've had a few [heavy defeats] in my career and it's just not the best thing in my debut in the starting side ... you get given a chance to prove yourself and it's just shattering.

"I was looking for a rope to hang myself. South Africa just played out of their skin. The first five minutes everything was going well but after that it just felt like we were running backwards all the time. It was probably one of those games where you want to wake up and just think this is a bad dream.

"I felt my defence is probably one of my strengths but there's probably just a few little technical things.

"I missed one tackle there, which cost us a try - personally that was probably the down point of my game, but I thought I was doing everything right."

The Wallabies' woes in this area are in contrast to the All Blacks as they head into a match where the Tri-Nations title is on the line and the Bledisloe Cup is delicately balanced at 1-all in the four-match series.

An All Blacks' victory would secure both pieces of silverware while the Wallabies can win the Tri-Nations and keep the Bledisloe alive heading into Hong Kong with a result going their way at Ballymore.

The All Blacks have opted for solidity at second five-eighths with Ma'a Nonu starting every test this year and certain to be at No. 12 against the Wallabies as long as he comes through unscathed against Samoa.

They also seem to be increasingly comfortable with the Nonu-Conrad Smith combination, a partnership that has been given more opportunities with alternative centre Richard Kahui switched to the right wing to accommodate a shortfall there.