Historically, ANZAC (Australian and New Zealand Army Corps) was an acronym devised by Major General William Birdwood's staff in Cairo in early 1915. It was used for registering correspondence for the new corps and a rubber stamp was cut using the letters A.&N.Z.A.C. Australia's Official War Historian of the Gallipoli campaign, Charles Bean, ascribed the origin of the acronym to a British Officer of the Army Service Corps, Lieutenant AT White. Another British Officer, Major CM Wagstaff, suggested running the letters together – ANZAC – as a code word for the corps.
After the landing at Gallipoli, General Birdwood requested that the position held by the Australians and New Zealanders on the peninsula be called 'Anzac' to distinguish it from the British position at Helles. Permission was also sought to name the little bay, where the majority of the corps had come ashore on 25 April 1915, ‘Anzac Cove'. The letters now were upper and lower case, indicating that the original acronym had already found a use beyond that of a military code word or corps designation. Not surprisingly, the word was soon applied to the men of the corps who became 'Anzacs'. There is substantial contemporaneous documentary evidence such as diaries and letters from those soldiers showing that they used the term in upper and lower case.
The Anzac Book, which was published in 1916, was written in 1915 by the Anzacs themselves, while still at Gallipoli. In their own writings and illustrations the word is frequently spelt as 'Anzac'.
By the time Charles Bean wrote his two volume official history of the Gallipoli campaign in the 1920s, the word 'Anzac', in upper and lower case, was well established. Indeed, the histories were called The Story of Anzac, not ANZAC. In the Glossary at the back of volume II, Bean outlined the various usages of the word during World War I. After Gallipoli, it was again used to refer to the two large Australian and New Zealand units in France and Belgium – 1st Anzac Corps and 2nd Anzac Corps. On the Western Front also, British soldiers used the term 'Anzacs' to describe the Australian and New Zealand soldiers, although strictly speaking the only men entitled to that description were those who had actually served at Anzac (Gallipoli). That service was proudly denoted by a brass 'A' on a man’s unit shoulder flash.
Department of Veteran Affairs website www.dva.gov.au
The earliest mention I can find of the word 'Anzac' in a local paper is from the Lang Lang Guardian of December 15, 1915. It is reproduced below. It is interesting also because it gives us an glimpse of the sort of fund raising activities that were being undertaken by local communities to raise funds for the war effort.