Joint leader with England's Nick Dougherty and Spaniard Garrido with a round to play, the 2008 Challenge Tour graduate did not drop a stroke in shooting a four- under-par 67.
Duncan Hodge's drop goal on the stroke of half-time proved crucial for the Reivers and the Welsh were unable to match their opponents forward power and pace in the second half.
Contepomi added the extras to give Leinster a healthy 9-17 lead, but on the stroke of half-time Connor sent over a drop-goal to reduce the deficit to five.
Sterne became the first player since Tiger Woods in 2002 to drop just one shot over 72 holes in a European Tour event when he cruised to a seven-stroke victory over Schwartzel at the Joburg Open, having led from the start.