The Macquarie Dictionary is usually considered the most reliable source for Australian English words, and Australian usage.
There are a range of editions in print, as well as the (paywall) online version.
Another option is the Australian National Dictionary, although it focuses on purely "Australian" words and their first usage. Rather than the real language used in Australia which (like most forms of English) is a mix of British, American and local words.
For basic checking of spelling, your word processor may have an "Australian English" option. Open Office does, and most of the words that are missing (like arvo) are probably a bit colloquial for the web anyway.
For a style guide, see the ABC Radio National Style Guide. Covers very Australian things like the way the Labor Party is spelt like that, and labour is the way the work is spelt.