GeorgeV
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.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians