Sim shot a 68 to match Allenby at 10-under 206 on the Huntingdale course.
Ashley Hall (68) was two strokes back, and Rod Pampling (70), Anthony Summers (71) and Englands Webster (70) were another shot behind at 7 under. Australian players held five of the top six positions on the leaderboard.
Allenbys 3-wood shot on the 535-yard seventh landed on the front of the green and ran about 25 feet direct before it hit the flagstick and droped in.
Its a good way to kick-start the round, Allenby said. To hole a 3-wood was pretty awesome. Michael and I both played pretty well.
As the gallery cheered, Allenby raised his arms, blew a kiss and pumped his fists.
It was about as pure as it looked, Allenby said. The wind was just off the right. I knew I didnt have to smash it. I knew I just needed to hit it solid.
I was just going straight at (the hole) and it did exactly that. It went straight at it. It landed perfect.
With the green slightly above where Allenby was standing on the fairway, he could not see the ball roll in. But the crowds reaction let him know.
I saw it land and I saw it track towards the flag and it just sort of disappeared and then I heard the roar and obviously it had gone in, he said.
Allenby said he had to alter his club selection to counteract his pumping adrenaline on the following hole.
If I win tomorrow, I can say that that shot won me this tournament, Allenby said.
Second-round leader Tim Clark was seven strokes back after a 76.