Australia’s Integrated Marine Observing System (IMOS) has been collecting sustained ocean observations for 20 years to help understand and manage Australia’s marine estate. IMOS operates observing equipment throughout coastal and open oceans, making all of its data accessible to the marine and climate science community, other stakeholders and users, and international collaborators. IMOS data holdings include long time-series of essential ocean variables including physical, biological, biochemical and atmospheric variables which are used in a range of applications which are crucial to understanding patterns and trends. Ocean observations and model outputs play a critical role in supporting decision-making in a wide range of fields, including fishing, aquaculture, shipping, oil and gas, offshore energy, maritime safety, defence and resource management. Long-term, sustained ocean observations also support our understanding of how climate change is affecting critical ecosystems and species. Therefore, there is an integral link between ocean observing infrastructure, research, and effective decision-making to improve ocean management and sustainable use. Understanding the state and trends of our oceans is critical to defining and measuring change in our ecosystems now and into the future.