i'm not sure of the exact dates/episode numbers as i was not a viewer in 2005, but i know that Bec Hewitt left as she was pregnant, and perhaps asked to leave the show earlier than she had expected/agreed to. i can imagine it must be difficult to juggle the last few months of pregnancy and a full-time job, such as acting in a soap opera!
so anyway, as far as i know, Ella Scott Lynch was cast at the last minute to fill in for the character's last few months on the show. she would have known full-well that it wouldn't be a long-term role.