It has now reached 553 accounts - which has also forced me to start a new Twitter list due to their 500 follower limit.
You can view the spreadsheet embedded below, or access it at: http://spreadsheets.google.com/ccc?key=0Ap1exl80wB8OdHNKVmQ5RVlvQWpibDAxNHkzcU1nV2c&hl=en
You can subscribe to the listed australian government accounts using my Twitter lists:
Australian-gov-tweets (500 accounts)
Australian-gov-tweets2 (my new 'overflow' list)
I haven't yet created by level or by state lists yet - but will do so when I have time.
If you're aware of any Australian government Twitter accounts I've missed, or if your agency starts a new account, please update the list directly, or let me know so I can update it (my main issue in maintaining the list's currency is that government agencies are poor at telling people about their new social media accounts).
Please look at the Stats and By population tabs in the spreadsheet for the list to see how many accounts there are by category, state and government level, how long the average Twitter account name is, when accounts were started and the most popular months for starting accounts, plus the ratio of Twitter accounts to population at state and national levels.