Left-hander Cullen eagled the 304 meter par-4 11th, holing out from 59 meters 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.
Kieffer was also happy with his form on his first visit to Melbourne.
"I love it, it's great. It's testing every part of your game," he said. "My expectations were very low because it's 10 hours time difference. It is quite a lot and obviously the preparation was not how it should be but it is a good learning experience."
Zimbabwe's Brendon De Jonge and Australia's Peter O'Malley had 68s and were in a group of eight players at 3 under.
Jarrod Lyle, returning to play after his second bout with leukemia, opened with a 72 and was level with Vijay Singh at 1 over.
After an emotional start, Lyle settled into a 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 1st 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.
