Australia's growing casualty count is boosting South African confidence, despite the home side holding the whip hand in the Boxing Day Test.

But on a day two with injuries aplenty, the most surprising one had nothing to do with the play, as Proteas paceman Anrich Nortje was knocked over by Spidercam.

The wire-guided Spidercam, which hovers above the players, went too low and collected the paceman from behind as he was fielding.

Nortje was able to bowl at the start of day three, but he was monitored for soreness in his left shoulder and elbow, which took the impact as Nortje was knocked to the ground on day two of the Test.

The camera hit Nortje at some pace, but he was on his feet immediately and was able to keep bowling.

While Nortje had to leave the field later, it was because of cramp as the players roasted in the mid-30s conditions.

Spidercam was not used for the rest of the day and its operators spoke to the South Africans, with the incident blamed on human error.

"I don't think it should be travelling head-height."

The tourists trailed by 197 runs after day two at the MCG, and their last seven Test innings have all finished below 200.