Create this file and name it fc-australian.def. Put it in the same directory as your test file for experimenting with it; if the tests are successful, move it in some place where TeX looks for input.
% File name: fc-australian.def
% This file has been obtained by modifying
% fc-british.def in the fmtcount bundle
\ProvidesFCLanguage{australian}[2013/06/20]
\FCloadlang{australian}
\let\@ordinalMaustralian\@ordinalMenglish
\let\@ordinalFaustralian\@ordinalMenglish
\let\@ordinalNaustralian\@ordinalMenglish
\let\@numberstringMaustralian\@numberstringMenglish
\let\@numberstringFaustralian\@numberstringMenglish
\let\@numberstringNaustralian\@numberstringMenglish
\let\@NumberstringMaustralian\@NumberstringMenglish
\let\@NumberstringFaustralian\@NumberstringMenglish
\let\@NumberstringNaustralian\@NumberstringMenglish
\let\@ordinalstringMaustralian\@ordinalstringMenglish
\let\@ordinalstringFaustralian\@ordinalstringMenglish
\let\@ordinalstringNaustralian\@ordinalstringMenglish
\let\@OrdinalstringMaustralian\@OrdinalstringMenglish
\let\@OrdinalstringFaustralian\@OrdinalstringMenglish
\let\@OrdinalstringNaustralian\@OrdinalstringMenglish
\endinput
%%
%% End of file `fc-australian.def'.
Now the example file
\documentclass{article}
\usepackage[australian]{babel}
\usepackage{fmtcount}
\begin{document}
\numberstringnum{100}
\end{document}
runs.
Note This will give errors with LuaLaTeX on TeX Live 2013, but it's not related to fmtcount. The problem is probably related to the recent changes in babel management. It should be reported to the people in charge. It disappears if I modify the entry in language.dat.lua for ukenglish with the addition of the synonym australian.
['ukenglish'] = {
loader = 'loadhyph-en-gb.tex',
lefthyphenmin = 2,
righthyphenmin = 3,
synonyms = { 'british', 'UKenglish', 'australian' },
patterns = 'hyph-en-gb.pat.txt',
hyphenation = 'hyph-en-gb.hyp.txt',
},
fc-australian.defby modifying the existentfc-british.def– egreg Jun 20 '13 at 13:36