You can use the following approach:
1) Create en_AU resources
2) check if your current locale is en_NZ
NSString *currentLanguage = [[NSLocale preferredLanguages] objectAtIndex:0];
// or
NSString *langID = [[NSLocale preferredLanguages] objectAtIndex:0];
NSString *lang = [[NSLocale currentLocale] displayNameForKey:NSLocaleLanguageCode value:langID];
NSBundle *xBundle;
3) in case en_NZ:
// find the path to the bundle based on the locale
NSString *bundlePath = [[NSBundle mainBundle] pathForResource:@"Localizable" ofType:@"strings" inDirectory:nil forLocalization:@"en_AU"];
// load it!
xBundle = [[NSBundle alloc] initWithPath:[bundlePath stringByDeletingLastPathComponent]];
4) in case other than en_NZ:
xBundle = nil;
5) use in your code:
NSLocalizedStringFromTableInBundle(@"yourStr2Localize", nil, xBundle, nil);