Check your elements, sounds like the lower might be out, also could be your fill tube broke in heater, you will have to drain heater and disconnect pipe and El. and bend it over to take out tube and check it, let me know.
In electric heaters usually it’s the element (especially the lower as Retired said) or one of the thermostats (primary or secondary). For the cost of either, I also recommend changing both elements at the same time and same for thermostats while you are already at replacing one of them, that way you know both will be new and last.
Check the amperage being drawn through your thermostat to the element – it is possible and I have found it to be an increasing problem since all this deregulation of the Electrical Eneregy Suppliers has happened. Your problem given you are in Auckland should not be the result of hard water corroding your element but it is possible that the ripple control unit feeding the power to your thermostat then to your element is faulty. Power Companies control the power supply to electric water heater via the ripple control module on your meter board so that at peak times they can shut off the power to water heaters. You can do an ohms test on the element which will show if that is still ok. Hope all this helps.
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