This book constitutes the refereed proceedings of the 4th Australasian Conference on Information Security and Privacy, ACISP'99, held in Wollongong, NSW, Australia in April 1999. The 26 revised full papers presented were carefully reviewed and selected from a total of 53 submissions. The book is divided in topical sections on Boolean functions, key management, cryptanalysis, signatures, RSA cryptosystems, group cryptography, network security, electronic commerce, address control, and odds and ends.