cant find Mary Squire

Hi all, I've posted about this on the Brick Wall site too, but this really is an Aussie problem. Kind of. My 3x Great Grandmother, Mary Squire was born either in Devon or Cornwall England, no idea when or where she got here, although on 30 Nov 1844 in Scotts Church in Sydney she married James Bennett, I have a fully copy of the wedding certificate, and no family listed, and the witness names are not familiar. They moved to Melbourne, and had 3 children, the latest Sarah Jane Courtney Bennett born 1848 in Brunswick, after that we here nothing, until James remarries on 1 Jan 1851 to an Emily Reeves, they have 2 children that die. Then James dies in 1857 and Emily gives the details, including the wrong names of his first lot of children. Thats it. James is buried at Melbourne General Cemetery Carlton, no mention of Mary at all. I've looked and looked and looked. I think she might have a mother called Courtney because of her daughters name Sarah Jane Courtney - just a guess, but that's it. Any idea's anyone?