Due Diligence at a Crossroads: The Old Road, the New Road, and the Bridge Between

Speaker: Dr Penelope Ridings, International Law Commission

Lecture summary: In the last several decades, scholarly views of due diligence in international law have shifted from due diligence as a primary obligation under customary international law, to due diligence as a standard of conduct attached to a primary obligation. Thus, for example, due diligence is required to meet a State’s obligation of protection (of the environment) or of prevention (of genocide). The International Court of Justice in its Advisory Opinion on Climate Change adopted such an articulation and stated that due diligence is a standard of conduct and States have a duty to prevent significant harm to the environment by acting with due diligence. The Court not only reinforced the importance of the customary international law obligation not to cause significant harm to the environment but placed this within the ‘no harm’ principle, as expressed in the Corfu Channel case. However, the Court did not expressly articulate whether there was a broader obligation of due diligence that applies not only to the prevention of environmental harm, but also to the prevention of other harms to the rights and interests of States. Due diligence is thus as a crossroads. Has the ICJ essentially sought to bridge the gap between on the one hand the notion of due diligence as an obligation on a State not to permit activities subject to its jurisdiction or control which causes harm to the rights and interests of other States, and on the other hand the notion of due diligence as a standard of conduct attached to a primary obligation? Has the Court opened the door to finding a general customary international law obligation not to cause harm to the rights and interests of other States? Or has it confined due diligence to its status as a standard of conduct attached to a primary obligation? This lecture will discuss this pivotal point which is central to the elucidation of the foundation and scope of the due diligence obligation under international law.

Dr Penelope Ridings is a Member of the International Law Commission and New Zealand Barrister practising in the field of public international law. In 2025 she was appointed the ILC Special Rapporteur for the topic ‘Due Diligence in International Law’. She was formerly New Zealand’s Chief International Legal Adviser in the Ministry of Foreign Affairs and Trade and a New Zealand diplomat. She was Agent for New Zealand before the International Court of Justice in Whaling in the Antarctic: Australia v Japan, New Zealand Intervening and before the International Tribunal on the Law of the Sea in the Request for an Advisory Opinion submitted by the Sub-Regional Fisheries Commission.

After moving to the New Zealand Bar, she has advised governments and international organisations on public international law including law of the sea, fisheries, environmental law, trade and investment, international security and international dispute settlement. She was Chair of the 2025 arbitration under the EU-UK Trade and Cooperation Agreement (UK-Sandeel) and Chair of the WTO appeal arbitration China – Enforcement of Intellectual Property Rights under the Multi-Party Interim Appeal Arbitration Arrangement. She has served on several ICSID ad hoc Annulment Committees, including as Chair, and as an independent panellist in disputes before the WTO. She has lectured in international law and contributed to several books and written articles on various aspects of international law.

This lecture was delivered on 13 February 2026 and is part of the Friday Lunchtime Lecture series.