Besides being a senior consultant to Organisation for Economic Co-operation and Development for 20 years, Ellyard has been a Senior Adviser to the United Nations for more than 30 years, including to the 1992 Earth Summit where he was a senior advisor (the only Australian) on both the climate change and the biodiversity conventions. He also advised a number of International conferences including the United Nations Conference on the Human Environment (1971), The UN Conference on Human Settlements (1976). At other times he has been a senior consultant to the UNEP, UNDP and UNESCO.