The invention relates to a method of determining water level in a humidifier chamber that is part of a humidified gases delivery apparatus and system. The method comprising the steps of delivering power to a heater plate, varying the power delivered to the heater plate, measuring the rate of change of temperature and determining the level of water based on the heating characteristics of the volume of water within the chamber, in particular determining the level of water within the chamber based on the rate of change of temperature and the supplied power.