Helen, the Vic Registry marriage indexes online have 5 Catherine Cain marriages between 1921 and 1940, with 3 of these between 1921 and 1930. One is in 1925, which would fit quite well if she were the Cath Cain arriving in Vic in 1923.
I think local Vic indexes on fiche or CD would give more information, such as who she married?

Liz, none of these have father's name "John" recorded, nor mother's name "Julia", according to the online index.

Sue