I am not sure if you are still looking for your father and his family but we may be related. My father's name is James (Semisi) Francis Williams who migrated to NZ in the 1950s. He passed away in 1974. I do know that families with the surname Williams in Tonga are linked to Samoa and Fiji.
Contact me and maybe if you are still hooked to this link, we can talk more to see if we are connected.
Maria Williams Meredith