Sim and Carballo (68) had a 36-hole total of 8-under 136 on the Moonah Links course that has twice hosted the Australian Open.
Four players were tied for fifth at 6-under 138, including Australian veteran Peter O'Malley (70), who was among five who shared the first-round lead. Australian Steven Conran shot a 67 while Americans Daniel Summerhays and Hunter Haas each had 70s and are tied with O'Malley.
New Zealand amateur Danny Lee, who won last week's Johnnie Walker Classic at Perth, missed the cut after rounds of 74 and 75. Lee, last year's U.S. Amateur winner, had a 5-over total of 149, missing the cut by five strokes.
Sim, who finished seventh at Perth, is a former member of the Australian Institute of Sport's golf program based at Moonah. He estimated he's played the Moonah Links course "between 35 and 50 times."
"I know the course better than anyone else in the field," Sim said. "We came down and played once or twice a week on the Open course. You know the shots to hit around the greens, you know where to bail it out, you know the breaks on the greens."
Sim turned professional in 2005 and the following year won on the Nationwide Tour and earned a promotion to the PGA Tour in 2007.
But a stress fracture he suffered in his back in 2006 led to a late start on the main circuit and a limited schedule of 17 tournaments. He had a medical extension last year but could only play five PGA events as well as some smaller tournaments toward the end of the year -- about 14 in all.
He's now aiming to rebuild his career.
"I'm riding a bit of a confidence wave right now," Sim said. "I'm just starting to believe in myself a little bit more."