On 3 September 1939 Prime Minister Menzies announced that it was his ‘melancholy duty’ to inform the country that Australia was at war. The stage was set for what was to become one of the greatest tests for Australia as a nation. At the outbreak of war Berryman held the position of GSO1 (General Staff Officer Grade 1) of the 3rd Division. The division, then under the command of Major-General E.A. Drake-Brockman, was based in Melbourne. Berryman's role as GSO1 made him the senior staff officer in the division ‘responsible for operational planning, intelligence, training and the preparation of orders and reports’. For all intents and purposes he was also the division's chief of staff, being the commander's ‘right hand man’, commander of the headquarters unit and responsible for the ‘coordination of all of its staff branches’. Berryman's GSO1 posting was supposed to be a precursor to a move back to London to attend the senior officers' course at the Imperial Defence College. Rowell, Lavarack and Drake-Brockman had been continually lobbying for such a posting on his behalf. However, the onset of the war in September cancelled all such appointments.

Meanwhile Berryman, like all officers, watched the government's reaction to the war with a great degree of professional curiosity. His initial reaction was one of despondency as the government failed to implement the army's mobilisation plans on which he and a team of other senior officers had worked exceptionally hard in the years leading up to the war.