The former NSW Blues and Knights halfback was invited to training by Wallabies attack coach Scott Wisemantel on Thursday.
Johns, who previously dropped in on Eddie Jones' England camp during their 3-0 whitewash series win in Australia in 2016, only spent 30 minutes with the Wallabies but his appearance went down a treat.
It's believed Johns had hoped to fly under the radar but Wallabies halfback Nic White let the cat out of the bag by posting a photo on his Instagram account.
Johns spent time with the entire team and passed on knowledge around the block plays in rugby league and union and what triggers to look for.
His appearance comes after the Wallabies failed to score a try against the Pumas during their 15-15 draw in Newcastle.
The Wallabies had their chances last weekend but failed to take advantage of two clear cut scoring opportunities.
But Dave Rennie's men came under criticism for their failure to close out the match, where they let a 15-6 lead slip from their grasps and were guilty of poor discipline and gifting possession away.
The draw means the Wallabies' hopes of winning some prized silverware have taken a massive hit.
Despite losing two Tests throughout the Tri Nations, Ian Foster's All Blacks can all but seal the trophy if they beat the Pumas with a bonus-point on Saturday in Newcastle.
The All Blacks are paying Winx-like odds to beat the Pumas despite Mario Ledesma's men recording a win and draw from their opening two matches of the Tri Nations.
Ledesma's Pumas also had a special rugby league guest provide some tips on Thursday.
Current NRL star and Blues halfback Nathan Cleary was spotted at Pumas training on Thursday.
"His kicking coach is in New Zealand and he couldn't come so he offered Nathan to come and give some pointers to the boys," Ledesma said on Thursday.
"I thought it was really nice of him to come and give us a hand.
"Being a big rugby league fan I was happy to have him."