Abstract: A sensor assembly for monitoring an internal condition of a transformer comprising: a body for location outside a tank of the transformer, the body housing an electronics assembly including body sensors coupled to a processor; a probe extending from the body for insertion into a tank of the transformer; a number of probe sensors coupled to the processor disposed at or toward a remote end of the probe for sensing conditions within the tank; wherein the processor is configured to process signals from the probe sensors and the body sensors to produce signals for assessing one or more of: transformer core temperature; transformer insulation state; partial discharge; mechanical state of transformer; chemical state of transformer.