Is your grandfather's surname a "matai title"? true birth "aiga"/family surname? If not, do you know other Samoan surnames in your family lineage?
MULIAVA listed with www.familysearch.org
are fairly new, the earliest being b.1918 all from the island of Upolu, [Western]Samoa. Manua is officially in the American Samoa region since 1900 what use to be [Eastern] Samoa or part of "the Navigators' Islands".
MULIAVA could have easily migrated to New Zealand yet there are no listings for that area.
I will ask elderly "aiga"/family members, esp. from Ta'u, Manu'a, AS, the reason[s] for certain surnames disappearing.
Do you know which island your ancestors dwelled on?
Manu'a or the Manu'a Islands Group consist of three main islands: Ta'u, Ofu, and Olosega. Ta‘ū is where Margaret Mead conducted her dissertation research in Samoa in the 1920's. Maybe your ancestors were part of that research - hmm?http://en.wikipedia.org/wiki/Manua
Fa'afetai/Thanks for your thoughts and leading me to disappearing Samoan surnames. I will investigate.