Cape Town – This one’s a biggie, all right, for coach Rassie Erasmus … but captain Siya Kolisi will also come under an increasingly harsh spotlight if the Springboks go down to Australia on Saturday.

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Certainly any (already hugely unlikely) prospect of either team beating holders New Zealand to the Rugby Championship crown this year will only flutter further out of the window for the losing outfit in Brisbane.

Cynics will be tempted to brand the duel at Suncorp Stadium (12:00 SA time) little more than effectively the beginning of another more secondary, best-of-the-rest competition for 2018 in the southern hemisphere.

It is almost too unpalatable to contemplate just how far behind the eight-ball the Boks will be deemed to be, now almost exactly a year ahead of RWC 2019, if they get beaten comprehensively on Saturday by a Wallaby side who so recently haemorrhaged 78 points across successive meetings with the world champion All Blacks.

But that, of course, would be to take a glaringly bleak view in the lead-up.

There will be a resurgent sense of optimism, by contrast, if South Africa can beat the Australians on their own turf for the first time since 2013.

Erasmus has controversially made mass changes for Saturday, which may have triggered some sense of unease in the camp — either that or a constructive awakening of some sort.

Whichever proves to be the correct backdrop, particularly assertive leadership by Kolisi, something that will need to echo in his own performance, seems essential for this one.

Let’s not beat about the bush: the fact that his own positional role (the ever-vexing question of whether he is better suited to No 6 or 7 flank) remains a “merry-go-round” at this point serves as an indicator, even if only to some degree, that he cannot quite glue himself convincingly to a specific personal duty.

Kolisi is back on the open side for the Wallaby Test, after playing blindside in each of the pair of Argentina encounters, and I would argue (though there are dissenters on this score) that it is the slot where he has generally fared better in his currently 33-cap international career.

It has been written many times before, with validity, that Kolisi is a fairly complex hybrid between the two positions: certainly in build, he is almost too big for No 6 and yet also not quite strapping enough to be your classical blindside “wrecker”.

The No 7 drawback is aggravated when mobile Warren Whiteley is fielded (the status quo) as the eighth-man; Kolisi is likelier to come into his own as a blindsider when powerhouse Duane Vermeulen is wearing No 8.

More immediately, however, the 27-year-old must readjust – and fast – to the more stealing-based berth, where a few especially tough challenges face him on Saturday.

Just for starters the Wallabies, entirely predictably, will put out two ace fetchers in the same loose trio: their own skipper Michael Hooper, and another pocket battleship in David Pocock.

But if that already means the dice is a little loaded against South Africa’s lone designated fetcher, Kolisi, the snubbing of hooker Malcolm Marx – he is curtailed to the bench – only adds to the real danger of the Australians bossing the breakdown.

On several occasions in his exciting, still developing Test career so far, Marx has really been the Boks’ most inspirational source of ball-theft, getting those immense shoulders over the “pill” with zeal.

So if ever Kolisi wanted to put a major stamp on a key Test match, and in the face of some steep hurdles, Saturday seems a perfect opportunity.

Throughout this season, he has tended to deliver workmanlike performances a lot of the time, rather than truly earth-shattering ones personally.

He has burst to prominence by running some exquisite lines on attack, but also gone through noticeable phases of games where his general influence somehow seems more muted.

There is less reason, I believe, to query his captaincy.

For all the ups and downs of the Bok Test year so far, their ability to chisel out victories in certain major clashes – like the decisive first two in the England series – from positions of some considerable arrears on the scoreboard, suggests men with a healthily-beating collective heart.

And that, by extension, usually means a bunch on the correct wavelength with their leader.

But remembering that South Africa largely continues its reputation as a “loose-forward factory”, including through several overseas-based players who will only crank up their candidacies as the World Cup looms larger, Kolisi putting beyond any doubt his own presence in the Test trio from a pure playing-levels point of view would be timely this weekend.

*Follow our chief writer on Twitter: @RobHouwing