A humidifier heater base assembly has a heater plate with a thermally conductive portion and a perimeter portion around a perimeter of the heater plate. A resilient member has an inner part attached to the perimeter portion and an outer part adapted to provide a resilient perimeter flange around at least part and preferably the whole of the perimeter portion. The resilient member fixes the heater base to the humidifier by the resilient perimeter flange such that the heater plate and the inner part can move relative to the humidifier in a direction substantially transverse to the general plane of the heater plate. At least a portion of the resilient perimeter flange remains stationary relative to the humidifier.