I am almost to submit a PhD thesis at an Australian university. My 100 word summary of what I planned to do is far different from what I actually did. I think that acceptance into the PhD almost entirely has to do with previous coursework results and having organised a supervisor, who can also tell you what to put for the 100 words. In mathematical biology two people at UQ come to mind to contact: Phil Pollet and Hugh Possingham. Fellow students I know have had good experiences.