Spelling the word anzac using only lower case is a spelling error.
Interestingly, in Microsoft Word if you misspell ANZAC, the capitalised version will not be suggested, but only the version with the initial capital. In fact Microsoft Word will even autocorrect a misspelled capitalised variation of ANZAC, to be the word Anzac, with the initial capital letter.
Kelvin Eldridge
www.Australian-Dictionary.com.au
The preferred Australian English spelling.