Corrections Act 2004, s 108; Prison Operations Manual, C.01.02, 03
The prison will tell you about anything that has been sent to you in the mail and that they’ve withheld from you.
Depending on what it is, the item could then be:
dealt with under the rules for prisoners’ property – for example, money will be deposited in your P119 trust account and phonecards will be recorded in the “inwards remittances book”
returned to the sender
destroyed
kept as evidence of intimidation or a threat to prison safety or security, or of a criminal offence.