The ASX website lists the following:
7am - 10am Pre-open
10am* Open
10am - 4pm Normal Trading
4pm - 4:10pm Pre CSPA
4:10pm - 4:12pm Closing Single Price Auction
4:12pm - 5pm Adjust
5:00pm - 6:50pm Adjust ON
6:50pm - 6:59pm Purge Orders
6:59pm - 7pm System Maintenance
7pm - 9:30pm Close
9:30pm - 2:25am System Unavailable
2:25am - 7:00am Close
So 9:30pm is the latest (GMT +10:00), I've seen announcements at this time before.