Hi Colin,

If the unit is still switched on it's likely to be one of two things, either the float valve is sticky and the unit is overflowing, or you have a proplem with the dump valve (stuck partially open or leaking past the piston). If the unit is switched off chances are the water valve is stuck open.

I'm not an A/C engineer but had to fix my own during the heatwave and learned pretty much all the in's and out's of an Evap system.

Hope that helps.