No. 2-ranked Scott made a bogey on the par-4 eighth -- his 17th hole of the day -- at Royal Melbourne to drop to 4 under and grab a share of second place with Germany's Max Kieffer.
Scott won the Australian PGA last weekend at Royal Pines in his first home event since winning the Masters. He will team with Jason Day for Australia next week in the World Cup, also at Royal Melbourne, then play the Australian Open the following week at Royal Sydney.
"A 67 around here is a good score," Scott said. "It was a bit of a slow finish but it was a good way to open up.
"I felt like I left a couple out there, but my game was in pretty good shape other than that, and four rounds like that might go a long way this week."
Left-hander Cullen eagled the par-4 11th, holing out from 65 yards with a 60-degree wedge. He followed with birdies at the 12th, 13th and 17th to open a three-stroke lead at 7 under but came back to the field with a bogey on 18.
"Any time you shoot 6 under around a course like this, you have to be happy," Cullen said.
Zimbabwe's Brendon De Jonge and Australia's Peter O'Malley had 68s and were in a group of eight players at 3 under.
After an emotional start, Lyle settled into the round and mixed three birdies and four bogeys.
"It was a feeling I thought I would never experience again," Lyle said. "The emotions overtook me on the first tee. It's something I always dreamed of, having my daughter at a tournament. It was great she got to see what I do.
"My dream is to come here and make the cut. After 12 holes things were looking grim but I fought like crazy."
Scott was leading after the early groups finished but was overhauled by Cullen, who fired an eagle on the 332-yard, par-4 11th and had six birdies but closed with a bogey at the 18th for a 65.
American Matt Kuchar opened with a 71.