Nick joined the PGA Tour of Australasia in 2009 after finishing 23rd at Qualifying School. He also competed on the Canadian Tour in 2010 where he recorded some good performances. On his home Tour in 2010 he recorded two top-10 finishes but returned to Qualifying School. He experienced a turning point in his career where a 5th place finish at the 2011 New Zealand Open earned him his biggest cheque to that point and also helped ensure his card for the 2012 season.
In 2012 he also earned status on OneAsia where he really stamped his mark with his debut win at the Indonesia Open.
In 2012 he also qualified for The Open Championship, his first appearance in a Major, but missed the cut. On the PGA Tour of Australasia in 2012 he recorded six top-10 finishes. The following year he went on to win his second professional tournament and first one home soil at the 2013 Queensland Open. In 2013 he suffered a set back having to withdraw from the Australian Open with appendicitis. This also stopped him from competing at Qualifying School for the Japan Tour. Looking for an international tour to compete on Nick attended Asian Tour Qualifying School at the start of 2014 earning full status for the season. November 2014 Nick won the BetEasy Masters held at Huntingdale that year by 1 stroke at 9 under.
Moving into 2015 Nick played extremely well to finish 2nd in the Oates Victorian Open championship.
Throughout 2017, Cullen has played consistently on the European Challenge Tour
- 2012 Indonesia Open winner.
- 2012 qualified for his first Major, The Open Championship via a playoff with Ashley Hall and Peter Senior for two positions via International Final Qualifying.
- Winner of 2014 BetEasy Masters
- Plays on the Challenge Tour