Australia have been dealt a huge injury blow on the first morning of their Test series decider against South Africa with Steve Smith ruled out with a neck injury.

Smith had been dealing with the issue since the second Test in Gqeberha but had appeared primed to play in Cape Town as his side looked to bounce back from defeat.

But having appeared in discomfort in the nets on match eve, Smith again struggled to bat properly with his neck heavily bandaged on the first morning, and was scratched from the Australia XI as one of four changes.

"He had another little try this morning but wasn't happy with how his neck was feeling, so we had to look elsewhere,” captain Pat Cummins explained of Smith’s injury. “We've got some guys really well-placed; they've been here working hard the last couple of weeks. A couple of injuries they're coming back from and they're now 100 per cent."

Smith had been forced to drop down the order in the second innings as Australia sought, and failed, to save the Test in Gqeberha.

Josh Inglis is the beneficiary of the 37-year-old’s absence and will bolster the middle order, with Cameron Green, Cooper Connolly and Nathan Lyon also returning to the side.

Matt Renshaw will move up to open alongside Travis Head with Nic Maddinson dropped, while Scott Boland is unfortunate to miss out for Lyon having been a standout performer in the second Test defeat.

The three-match series is tied at 1-1 in the first meeting of the two teams on South African soil since the Sandpaper-gate saga of 2018.

Australia elected to bowl first at Newlands with the hosts bringing in seamer Gerald Coetzee for an injured Anrich Nortje.