A great weed book that I can recommend for the central and southern sections of the east coast of Australia is Weeds of the South-east: An Identification Guide for Australia. It is written by F.J.Richardson, R.G. Richardson and R.C.H. Shepherd.
The second and fully updated edition of this book was released recently (2011) and I think it is brilliant. Great photos, easy descriptions (if you want them) and worth its cost if you are regularly needing to identify weeds in your area and not a true and passionate botanist. It is also easily available from online stores.
I live in south-east Queensland and it is certainly relevant for my area. And because of the geographic distribution of the book's subject matter you can also look at or learn about some of the weedy species that occur in the cooler parts of the continent.
Thumbs up for this publication!