MELBOURNE, November 14, 2013 (AFP) - Cullen, who won last year's Indonesia Open, holed an eagle and six birdies on his way to a 65 and lead world number two Scott and Germany's Maximilian Kieffer by two shots.
Left-hander Cullen, 29, who turned professional only four years ago, surged to the outright lead with a purple patch from holes 11 to 13, first chipping in for eagle then notching two birdies to climb from two-under to six-under.
"I hit it really well and putted pretty solid. I pitched in on the 11th which always helps," said Cullen, who holed his eagle from 59 metres with a wedge.
He briefly reached seven-under before finishing with a bogey.
Scott headed to the practice range straight after his round, but was content with his score.
While he missed some early birdie chances and tailed off late, after reaching five-under with five holes to play, he was pleased that he capitalised on some fine mid-round ball striking.
He birdied holes one to four, having started on the 11th.
"You just need a few good holes to have a good score and I was patient," Scott said.
"I was trying not to get frustrated because I felt like I had left some opportunities out there.
"That is all you need to have a good round, a hot 20 minutes."
Scott, who won last week's Australian PGA on the Gold Coast, said since his breakthrough as the first Australian to win the US Masters in April, he had been determined to keep riding the momentum.
"My big thing after winning the Masters was to keep my head down and practise hard and use the confidence of winning a big tournament," he said.
"I am trying to keep my momentum going and strike while the iron's hot."
Kieffer carded an eagle and two birdies in his unblemished opening round to share second place with Scott,
On his first visit to Australia, Kieffer praised the Royal Melbourne course.
"I love it, it is great. I think it is testing every part of your game," said Kieffer, who before Thursday had only played the front nine holes of the course.
"My expectations were very low obviously the preparation was not how it should be but I think it is a good learning experience."
Zimbabwe's Brendon De Jonge was among a large group on three under after five birdies and two bogeys.
American world number eight Matt Kuchar and US-based Australians Geoff Ogilvy and Marc Leishman were all even with the card, while cancer survivor Jarrod Lyle shot 72 in his emotional return after 20 months out of tournament golf.
Fiji's former world number one Vijay Singh was in a tie for 38th after a one-over 72 with five bogeys outnumbering his four birdies.
Leading scores after the opening round of the Australian Masters at Royal Melbourne (par-71) on Thursday (Australia unless stated):
65 - Nick Cullen
67 - Maximillian Kieffer (GER), Adam Scott
68 - Anthony Brown, Stephen Leaney, Peter O'Malley, Ryan Fox (NZL), Brody Ninyette, Brett Rankin, Nathan Holman, Brendon De Jonge (ZIM).
69 - Andre Stolz, Peter Wilson, Jason Scrivener, Clint Rice, Craig Hancock, Gaganjeet Bhullar (IND), Terry Pilkadaris, Jason Norris, Matthew Griffin, Matthew Millar, Bryden MacPherson
70 - Stephen Dartnall, Cheng-Tsung Pan (TPE), Josh Geary (NZL), James Nitties, Andrew Tschudin, Peter Cooke
71 - Bradley Lamb, Daniel Fox, Rohan Blizard, Marc Leishman, Matt Kuchar (USA), Matthew Guyatt, Adam Bland, Geoff Ogilvy, Mark Brown (NZL).
Selected scores:
72 - Vijay Singh (FIJ), Jarrod Lyle
73 - Anirban Lahiri (IND), Jin Jeong (KOR)
75 - Siddikur Rahman (BAN), Jun Seok Lee (KOR), Casey Wittenberg (USA)
79 - Thomas Petersson (SWE)