How do I do a search for all those who enlisted in WW1 from the Mount Isa district?
1 - Apparently Mount Isa wasn't established until 1923, so I am not sure what you consider to be part of that district.
2 - With "from the district" do you mean persons who were born in the area or persons who enlisted there?
I get 126 hits when searching on the exact phrase Place of Enlistment - Cloncurry and date 1914 to 1918.
You could substitute other place names from the required area or use "Place of birth - NNNNNNN" and see whether you get anything useful.