Most departments of the Australian Government use the Style Manual for Authors, Editors and Printers as their style guide. Many other organisations also use it.
It covers the planning process, writing and editing, design, illustration usage, the legal aspects of publishing, and final product evaluation.
I have found the writing and editing section most useful and recommend it for both writers and editors. In particular, there are in-depth sections on grammar, spelling, punctuation, capital letters, shortened forms, citations and references.
UPDATE: Since 2020 the the Australian Government Style Manual is now online and free to access here: Australian Government Style Manual
To find other recommended resources for writers and editors, check the resources page.