The problem is with the asynchronous controller. Instead, you should set the default culture for all threads:
app.Use(next => context => {
CultureInfo.DefaultThreadCurrentCulture = CultureInfo.GetCultureInfo("en-AU");
CultureInfo.DefaultThreadCurrentUICulture = CultureInfo.GetCultureInfo("en-AU");
return next(context);
});
You can simply put these lines at the beginning of the Configure method:
CultureInfo.DefaultThreadCurrentCulture = CultureInfo.GetCultureInfo("en-AU");
CultureInfo.DefaultThreadCurrentUICulture = CultureInfo.GetCultureInfo("en-AU");