The wounded Wallabies must make key Tri Nations changes, but altering their fitness work won't be one considered as they attempt to overcome their bad habit of surrendering leads.

Australia, to meet unbeaten South Africa in a must-win clash at Subiaco Oval on Saturday night, have been criticised for a lack of second-half energy in their two Bledisloe Cup defeats to New Zealand.

In both matches, the relentless All Blacks overcame halftime deficits to triumph, including their 19-18 win in Sydney last weekend after trailing 12-3 at the break.

But Wallabies assistant coach Jim Williams on Wednesday was immense in his praise for the fitness levels of the Australians, citing their inability to win the territorial and possession battles, particularly due to inaccurate kicking, for the loss.

As in their 22-16 defeat at the hands of NZ in Auckland, and certainly the recent 29-17 loss to the Springboks in Cape Town, Australia spent much of the second half tackling themselves to a standstill.

"I think maybe the fact we had to defend so much last week that some people thought the boys weren't fit," Williams said. "That does take a lot out of you.

"We were running a lot and they were stretching us wide but the boys were defending magnificently.

"I thought the fitness was excellent and that showed through in the latter parts of the second half, we just had to defend so much in the 22 and in our own half.

"We certainly don't see fitness as an issue at all."

Williams said the team's fitness levels had improved significantly since Robbie Deans took over as coach last year and identified them as a key area for work.

"I thought there's been excellent strides through that area," Williams said.

With Nathan Sharpe (shoulder) and Berrick Barnes (concussion) sidelined, Deans will be forced into at least two changes when he names his side on Thursday, and possibly more with Rocky Elsom (ankle) and James O'Connor (cork) still in doubt.

Elsom was on Wednesday walking around without the moon boot he had been wearing and Williams was quietly confident both would be able to train in the morning.

Mark Chisholm could be rushed straight back into the second row for Sharpe ahead of Dean Mumm while Ryan Cross is likely to play at outside centre, forcing a backline reshuffle.

Halfback Luke Burgess, prop Al Baxter and both wingers - Lachie Turner and Drew Mitchell - are also under pressure to hold their positions, but Deans may be reluctant to tinker too much.

Williams attributed the side's fade-outs to a lack of experience.

Australia have never been renowned front-runners. Even when Williams played his 14 Tests around the turn of the century under John Eales, their success was built on winning big games at the death.

"The experience of the players was a lot different - Eales, (George) Gregan, (Stephen) Larkham, (Matt) Burke and all those guys - and we're building towards having that," Williams said. "We blooded 13 players last year and there's a lot of young guys.

"They're still learning the trade."

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