Census night is Tuesday August 9, but you can save yourself a lot of hassle by completing the online eCensus version instead ahead of time -- no working through questions you don't need to look at on the 18-page paper form. Most of the online process is fairly self-explanatory, but here are 10 issues to bear in mind.
Yes, you have to participate in the census. Participation in the census is compulsory for all Australians. As the data is used to help determine allocation of government funds, it's also an important part of participating in a democracy. Individual data from the census is not made public. (There's a question where you can opt-in to have individual forms made publically available 99 years from now. Future family historians will be grateful if you do, but the choice is yours.)
You can't simply go online and fill out the form. In order to use the eCensus, you'll need both an eCensus number and a census form number, which are supplied by your local census collector. If you're home when the collector drops in, you can simply get the envelope; if you're out (as I was), your collector will leave both a paper form with a number and an eCensus envelope.
You can get extra numbers for extra people. Up to 10 people can fill in one census form for a dwelling. However, you can ask to have your own form to submit so your details remain private. This could be useful in a share house, especially if you don't want your housemates knowing your approximate income.
Use the information buttons if you're not sure about a question. Each question includes an information button (a blue letter i) which provides more background. Click on it to read the background, and again to go back to the question.
You'll automatically skip past irrelevant questions. One big advantage of the online census is you don't have to read through every question on the paper form. Options that aren't relevant to you will be automatically skipped over.
Think carefully about the religion question. The question about religion isn't compulsory; if you do answer, you can choose from a list of common religions, specify your own, or answer 'No religion'. You should answer the question in a way that reflects your beliefs honestly, not enter "Jedi" as a joke or pick a particular religion that reflects your upbringing rather than your current belief. As Gizmodo has pointed out, data from this question is used to fund public services, so it should accurately reflect modern Australia.
The process isn't too time-consuming. I did the entire form in about 15 minutes. I'm a fairly fast reader, but I was also stopping to take notes and screenshots.
The internet usage question is disappointing for geeks. There are only four choices on offer: broadband, dialup, mobile or none. Many Lifehacker readers probably have multiple options from this list, but this is not the time to boast about how you alternate between Next G and ADSL2+.
You can submit before August 9. While the census form should reflect who is in your household on the night of August 9, the system will let you submit ahead of time. Given that the servers are likely to be overloaded on the night, there's a lot to be said for getting in before that.