Have you been paying Superannuation since you started? If so, there is nothing to buy back. However if you weren't paying Superannuation because you were temporary for example - then you can buy that back.
I think what you really mean is buying Notional Service. This is a way of having extra years when you reach retirement. You need to know what age you can retire at, which varies depending on your job and when you started.
For example I could retire and receive my pension at 60, but I don't have to retire til 65. I need 40 years for a full pension. The longer I stay, the more years service I'll have. I am also paying Notional Service which has been calculated based on retiring at 65, but I could've chosen to pay more so I could retire at 60 instead. I pay this monthly until I retire. If I choose instead to retire between 50 and 60, I will receive an Actuarially Reduced pension, but it is impossible for me to have 40 years service before I turn 60.