Secure management of Australia's commercial critical infrastructure presents ongoing challenges to owners and the government. Currently a high-level iriformation sharing collaboration between the government and business manages complex security issues, but critical irifrastructure protection also lacks a scalable model exhibiting the overall structure of critical infrastructure at various levels, sectors and sub-sectors. This research builds on the work of Marasea and Warren (2003) to establish a representative model of Australia's critical irifrastructure; discusses the boundaries between critical infrastructures, and considers the existence andpotential irifluence ofcritical irifrastructure relationships.