This LNCS-volume set in constitutes the refereed proceedings of the 31st Australasian Conference on Information Security and Privacy, ACISP 2026, held in Perth, WA, Australia, during July 6?9, 2026. The 69 full papers presented in this volume were carefully reviewed and selected from 260 submissions. The papers are organized in the following topical sections: Part I: Applied Cryptography and Cryptanalysis; AI, Machine Learning, and Trustworthy AI Security; Network, System, and Software Security; Blockchain, Smart Contracts, and Decentralized Security; Privacy, Authentication, and Cyber-Physical Systems Security. Part II: Applied Cryptography, Encryption, and Cryptanalysis; AI, Machine Learning, and Trustworthy AI Security; Privacy, Trust, and Secure Systems. Part III: Applied Cryptography and Post-Quantum Security; AI, Machine Learning, and Adversarial Security; Privacy, and Secure Distributed Systems; System Security, Privacy, and Secure Computing.