[1] As well as his medical practice, Begg was working at the Wellington District Hospital as honorary surgeon to the children's ward.

[2] Arriving in Egypt in early December, his command underwent training and saw brief service against the Turks at Ismâ'ilîya in February 1915.

[3] During the landing at Anzac Cove on 25 April, Begg was involved in surgical work on casualties evacuated to hospital ships.

The ships became overwhelmed with patients and on 28 April Begg landed at Gallipoli and set up a dressing station on the beach[1] which became operational the next day.

[1] Concussed and with an injured knee, he was admitted to the New Zealand Field Ambulance Hospital for a week before returning to duty.

Assembled in Egypt[6] before being transported to the Western Front in France,[7] Begg was the division's assistant director of medical services.

In October 1916, Begg became the deputy director of medical services (DDMS) of II ANZAC Corps, which was commanded by Lieutenant General Alexander Godley, and included the New Zealand Division.

[5] Among his initiatives was the establishment of a corps-level training school for its medical officers, a facility soon copied across the British Expeditionary Force.

[1] However, after the failure of the Battle of Passchendaele in October 1917, Begg wrote a detailed report on the II ANZAC Corps arrangements for evacuation and care of wounded and how these could be improved for future operations.