Largely forgotten as an Australian option in the five-day format after last playing a Test three years ago, Agar shone with 5-103 as SA were dismissed for 314 ahead of an early tea on day three.

The visitors then rammed home the advantage at Karen Rolton Oval, finishing the day at 2-195 - an overall lead of 362.

Shaun Marsh thrashed an undefeated, run-a-ball 102 while his former Test teammate Cameron Bancroft added 71.

Left-arm orthodox spinner Agar ran through the SA line-up just as a different allrounder - Queensland medium-fast bowler Michael Neser - put his name up in national lights with a century to go with his five wickets at nearby Gladys Elphick Park.

Agar routed the SA middle and lower order with key batsmen Travis Head (31) and Harry Nielsen (10) among his scalps.

"He's a really clever bowler and he uses his skills - he doesn't really miss at all," said teammate Cameron Gannon, who believed the allrounder would come into the Test mix.

"And, as we've seen, he can deliver with the bat."

He had also scored 116 not out, coming in at No.6 in WA's first innings of 5(dec)-481.

A short-form regular at national level, Agar initially came to prominence back in 2013 when he became the highest scoring No.11 in Test cricket with his 98 against England on debut.

But only three more Tests came his way as Agar didn't quite turn into the front-line spinner the Australian side had hoped for.

Six years younger than incumbent tweaker Nathan Lyon, he'll stay on the Test team radar, particularly if he can drive down his first class bowling average of 41 and continue to contribute with the bat.

For the home side it turned into a disappointing day after resuming at 1-178.

Jake Weatherald's (105) dismissal in the first over of the day set the scene for SA's collapse, Brad Davis (57) the only other significant contributor for the Redbacks on Monday.

Davis didn't seem particularly optimistic about his side's chances.

"I've played quite a few games on this ground now - and it's a game where you can't chase scores on the last day," he said.

Australian Associated Press