Bullock has been promoted to head up IBM's hardware business across the Asia-Pacific region.
He is being replaced by Glen Boreham, who first joined IBM in 1986 and was most recently in charge of the Telstra account.
Both appointments are effective immediately.
Under Bullock's stewardship, IBM has undergone significant growth in Australia and New Zealand.
He was appointed in 2001 to replace David Thodey, who left the company after 22 years to join Telstra.
Bullock oversaw the re-integration of the company's IT services and business consulting operations and played a key role in attracting international investment to Australia.
He also championed worforce diversity.
The new MD began his career in sales and spent a number of years working for IBM in London. During this time, Boreham was appointed vice president for IBM's sales and technical support in Europe. In this role he had responsibility for operational support in 124 countries covering 90,000 employees.
Boreham currently serves on the board of IBM Australia and is married with two children.