uPaged is not your employer - the hospital is your employer for each causal shift you work via uPaged.
Every casual nurse who takes a shift at a hospital must sign a casual contract with that hospital at the first time of their first shift so that you can get paid via their payroll and covered by their insurances. Even when you are there to work for only one shift and may never work there again, it has to be signed. You won't need to do this for subsequent shifts worked at that hospital.
A nurse cannot be an employee of a hospital and then pick up shifts at that hospital with any nursing agency. Because uPaged nurses become an employee of the hospitals they work at, they cannot return to these hospitals as an agency nurse, however, as a casual employee of that hospital via uPaged, you will be booked first and cancelled last by that hospital, and enjoy other benefits of being a casual employee of that hospital (ie: training and education, salary packaging, professional development etc).
Even though the hospital becomes your employer, you choose the shifts with the rate of pay you want, and you will always be a uPaged nurse as long as you continue to book shifts through the platform.
Since you are the hospital's casual employee, they will manage your payroll and pay your superannuation. Make sure you keep your superannuation details up to date so all of your payments are going into the one account.