The 22-year-old Coetzee (3-36) has genuine speed and had Jordan Buckingham (seven) caught in the gully and then dismissed Chris Tremain and Liam Hatcher for ducks.

In shattering Hatcher's stumps he also ensured Cricket Australia were bowled out for 226 in reply to South Africa's first innings of 347.

The Proteas were 0-9 at stumps in their second innings.

Unbeaten on 105 for Cricket Australia was former Australian under 19s representative Kellaway.

The stylish 20-year-old left-hander made his first-class debut for Victoria last month and came to the crease at 4-24 with Cricket Australia under assault from Proteas fast bowler Lungi Ngidi (3-26 off nine overs).

Jake Doran (78) and Kellaway added 143 for the fifth wicket and went after Coetzee initially in a superb counter-attacking partnership.

Kellaway was strong all around the ground and showcased power, touch and concentration in his innings.

"It was pretty cool. It hasn't really sunk in just yet," Kellaway told AAP.

"I was just trying to watch the ball as hard as I could, trust my processes and bat for a long period of time."

Ngidi took his early wickets in five fiery and fast overs.

He dismissed three of the top four Sheffield Shield run-scorers this season, with Tim Ward (one) caught down the leg side by wicketkeeper Kyle Verreynne with the first ball of his opening spell before he accounted for Sam Whiteman (11) and captain Peter Handscomb (nine).

Ngidi said he enjoyed bowling on the wicket.

"With the new ball you want to make inroads as early as possible and that is what we were able to do today," he said.

"With this wicket there is a lot on offer and that played into our hands."

Ngidi said Coetzee had a simple role to "run in and bowl quick".

"You saw at the end what he is capable of doing to the tail. If we can fine-tune that a little bit more I think he can be a very deadly bowler for us," he said.

The Proteas earlier resumed at 7-335 but soon lost Kagiso Rabada (10) and Verreynne (80).

Fast bowler Marco Jansen was then dismissed for a duck.

Middle-order batter Temba Bavuma did not feature in the first innings as he is nursing an elbow injury.

Captain Dean Elgar, who top-scored with 109 on day one, said after the close of play on Friday he hoped Bavuma would recover and get some valuable time in the middle before the first Test against Australia which starts on December 17 at the Gabba.

Cricket Australia XI fast bowlers Lawrence Neil-Smith (3-69) and Chris Tremain (2-42) impressed on day two after left-arm orthodox spinner Matt Kuhnemann (4-78) did the job on the first day.

Australian Associated Press