Here are the key things to check:
1. Ensure you are signed in to CloudShare (Google Apps) with your CEnet account (@syd.catholic.edu.au).
2. Ensure you are visiting the correct URL to access TD: https://www.teacherdashboard.com
3. Ensure you are an allocated teacher to a class in Teacher Dashboard. Primary School teachers can confirm this by clicking here to see a list of the classes you are allocated to in TD. Secondary school teachers should check with the TD Manager at their school.
4. If you are allocated to a class but still can't access TD It may be that you're trying to use multiple accounts in Chrome. That is, you've signed in with both your @syd account and some other account(s) and you periodically switch between these accounts.
The best way to run multiple accounts in Chrome is to use Chrome Profiles. Note that TD is known to have issues when you switch between accounts without using Profiles.
5. If you're still having problems, log out from CloudShare (Google Apps) and clear your Chrome cache completely (except passwords) before signing in again.