Strategic Environmental Assessment (SEA) and Cumulative Effects Assessment (CEA) are the focus of this research that investigated the evolving response in plans to the biologically undesirable accumulation of copper and zinc in a New Zealand estuary. Sources of metals are influenced by policies and plan provisions for land use, transport, stormwater management and boat moorings. During four decades of local government reform, plans responded to the scientific evidence of accumulation with increasingly sophisticated stormwater management. Despite the less explicit inclusion of SEA in New Zealand legislation, this research demonstrates for international audiences the concurrence of jurisdictional amalgamation; growing awareness and knowledge sharing across scientific, engineering and planning practitioners; and a steady improvement in plans to enable a marked slowing in cumulative effects (CEs) of urbanisation. A proposed Auckland Unitary Plan (PAUP) attempts to address some cumulative effects within watersheds now no longer divided by jurisdictional boundaries.