If you noticed almost all the males in the Melbourne scenes were young boys or old men.
Every one else was in North Africa.
Back then it was for adventure, king and country! Very few had any idea of the horrors of war that lay waiting, even after they came back, some with horrendous injuries, mental issues it was rarely spoken of, and when it was the truth of it all was never made public. Its only because of the advent of film, peoples personal diaries that some things ever make the light of day.
Australia was only a small country (still is some what) and to have so many men lost really hurt.
Some Australian Statistics
First World War 4/8/1914 -- 11/11/1918
416,809 enlisted AIF (includes AFC) -- 13.43 percent of the white male population and probably about half the eligible men.
331,000 enlisted and served overseas
61,720 died (all causes)
155,000 wounded (all services)
4,044 taken POW, 397 died while captive
1914-1918: four million