Van der Walt was part of the Bok Sevens side to Wellington and Las Vegas but will get his first taste of Super 14 rugby on the tour after receiving the SOS to fly to Australia.
Smith will leave in the early hours of Friday morning back to South Africa, and is due to arrive in Bloemfontein Friday evening to be with his family. Smith's father died unexpectedly after being in the hospital with Rift Valley Fever, contracted apparently in a recent outbreak in the Free State.
The Bok flanker had only joined his teammates on Monday night, after missing the previous two matches to be at the hospital with his father.
Smith's withdrawal from the team has also forced coach Naka Drotske to make a number of changes in his starting line-up.
Barend Pieterse returns to blindside flank where he covered for Smith in the past few games, with rookie Izak van der Westhuizen starting at lock in his Super 14 debut.
Nico Breedt, who was initially dropped from the side, comes back onto the bench for the game.