National Archives of Australia (google it, and search as a guest)has immigration records which should cover the period of immigration
I couldn't find Yiannis or Basilis immigration record.
There is a
CHRISTOFIDES Christo born 6 December 1917; nationality Greek
CHRISTOFIDES Christo: Nationality - Greek: First registered at Darwin - Northern Territory
I also could not find them on the electoral roll (it's what we use instead of census)
Did they come to Australia?