McNulty started the day with a two-shot cushion over Des Terblanche and managed cling to the lead after the Vryburg resident bogeyed the 18th to drop to eight-under for a one-under round of 69.
Terblanche shot a course record eight-under par 62 in Thursday's first round but was unable to sustain the momentum during the second round with a one-over 71, while McNulty tamed the tough conditions with a three-under 67.
During the third round McNulty opened a three-shot lead over Terblanche on the front nine only to see it whittled away on the back nine.
At the par-four 14th, McNulty hit his approach left of the green and the chip back settled three metres past the hole for an eventual bogey.
Terblanche took advantage at the very next hole, sinking a four-metre birdie putt to draw level with McNulty, who parred the hole from the greenside bunker, at nine-under.
But at the par-four 18th, Terblanche pulled his approach into the left rough, where the ball lay buried deep in the kikuyu and forced the drop.
'It was incredibly blustery out there,' McNulty said. 'On the 18th fairway, my caddie, Gary Todd, gave me a yardage of 121 to the flag. So I asked him, 'What's the message?' (121 being the number Vodacom cellular users dial to retrieve their messages). He just said, 'The message is hit the green, two putt and let's get out of here'.'
Their closest challenge comes from two veteran golfers at 205, five-under par. John Mashego carded a two-under par 68 while Hugh Baiocchi had to be content with a one-over 71, after playing the first 14 holes without a bogey. They upstaged the likes of Retief Goosen at two-under.