Callum is portrayed by actor Peter Mochrie.
Callum was in charge of the Primary Care Clinic in Shortland Street Hospital with TK Samuels. In 2007 he learnt of an affair his wife, Justine had with Chris Warner and they briefly separated, but they have survived worse than this in the past and eventually reconciled and enjoying life together with their energetic teens, Hunter and Sophie.
Callum has high ambitions and went power-crazy when acting CEO of the hospital while Chris was away. DHB appointed auditor Martha Riley known as Riley made recommendations to the DHB leading to Chris Warner's removal as CEO. Hone Ropata was appointed as CEO after inititally helping Riley with the audit. Hone turned down the role of CEO leading to Callum's appointment and implementing Riley's suggestions.
Callum had a brief affair with Riley in the couple of weeks before she left, and was caught red handed when his son Hunter came home to find Riley sauntering down the hallway from his parents bedroom. Justine learnt of the affair at Riley's leaving dinner when the DHB auditor broke down and announced her affections for Callum in front of everyone. Despite being upset, his wife took into consideration her own previous affair and they stayed together.
Callum tried winning over his staff, and in doing so locked horns with Craig Valentine, the head of E.D. Callum showed apparent favouritism to his wife, Justine in the funding of her department, over the new surgical unit being developed by Chris Warner. The staff referred Callum and his wife as Macbeth and Lady Macbeth.