This book constitutes the refereed proceedings of the 28th Australasian Conference on Information Security and Privacy, ACISP 2023, held in Brisbane, QLD, Australia, during July 5-7, 2023. The 27 full papers presented were carefully revised and selected from 87 submissions. The papers present and discuss different aspects of symmetric-key cryptography, public-key cryptography, post-quantum cryptography, cryptographic protocols, and system security.