Both sides had entered the encounter unbeaten but Joubert's double, along with tries from iSG Pumas scrumhalf Renier van Rooyen, No 8 Jeandre Rudolph and hooker Corniel Els, gave the men from Mpumalanga a bonus-point home victory, while flyhalf Chris Smith landed three conversions and two penalty goals.
The visitors managed to secure a bonus-point themselves, despite finishing nine points off the pace, by scoring four tries themselves through wingers Anthony Volmink and Edries Arendse, replacement Enver Brandt and No 8 Conway Pretorius, while flyhalf George Whitehead landed all four conversions.
Tafel Lager Griquas were out of the blocks in a flash when Volmink dashed over for the opening try after just six minutes. The iSG Pumas reduced the gap to four points when Smith landed the first of his two penalty goals before the home side secured the lead with their first try.
A sniping run by Van Rooyen resulted in the try which Smith converted to give the home side the lead by three points after 15 minutes, which they would not relinquish for the rest of the match. The home side extended their lead to eight points when Rudolph used his power to get over the line for a converted try as Smith landed the extra points.
But the visitors hit back immediately when Arendse finished off an excellent move making the conversion for Whitehead a little easier. Smith's second penalty extended the lead to six points before the Pumas rounded off the first half with their third try through Joubert's first effort, which Smith converted as the sides entered the tunnel with the Pumas leading by 13 points.
The iSG Pumas extended their lead in the second stanza to 18 points when Els barged his way over for the bonus-point try in the corner with Smith missing from wide but Tafel Lager Griquas hit back midway through the second period, with Brandt running through to score his side's third try, converted well by Whitehead to reduce the deficit to 11 points.
The men from the old mining town of Kimberley set themselves up for an exciting finish when Pretorius rounded off a good move to score his side's bonus-point try. Whitehead converted the effort with just 12 minutes left on the clock to leave Tafel Lager Griquas just four points off the home side.
But Joubert's second score of the game in the corner three minutes from time sealed the points despite Smith missing the conversion as the iSG Pumas maintained their unbeaten run and inflicted a first defeat of the season on Tafel Lager Griquas.
ISG Pumas - Tries: Morne Joubert (2), Renier van Rooyen, Jeandre Rudolph, Corniel Els. Conversions: Chris Smith (3). Penalty goals: Smith (2).
Tafel Lager Griquas - Tries: Anthony Volmink, Ederies Arendse, Enver Brandt, Conway Pretorius. Conversions: George Whitehead (4).