I researched about this in the past. AFAIK using hyphens are harmless unless you use it twice like ukrainians--au.com (scammers way). It won’t degrade the quality of a domain name.
But if both ukrainiansau.com and ukrainians-au.com are available then I will choose ukrainiansau.com. Because people already get used to use hyphenless domain names (not because of SEO).