Urgent and important correspondence from the University is usually sent by email to your University email account. Official correspondence will be sent to the address you supply to the Student Centre. Update your address and contact details through Sydney Student, which you can access via My Uni.
A Justice of the Peace (or JP) is a legally recognised witness for statutory declarations, and is able to certify signatures and copied documents. They offer this service for free. There are many JPs at the University of Sydney, but contact them beforehand if you want to use their services: https://sydney.edu.au/about-us/governance-and-structure/staff-directory.html#justice-peace
You can also find a JP near you by searching online for a Justice of the Peace in your area.
Download a statutory declaration form: ag.gov.au/Publications/Statutory-declarations
If you have lost or found any property on a University campus, contact Campus Assist on (02) 9351 7838, or speak to a Security Patrol Officer. There is also an online form you can fill out: sydney.edu.au/campus-life/safety-security