Ohaaki resource assessments are reviewed over its long exploration and production history. The recently IGA-adopted UNFC-2009 resource classification framework and geothermal specifications are used to categorize these resource assessments and analyse changes over time, in the context of changes to NZ regulations and electricity market. With the lessons from such a review, a proposal is presented to use the same UNFC-2009 based method to classify all NZ/Waikato geothermal systems as a basis of future resource management. Historic data is used to illustrate this and a proposal is presented how to gather data for an update.