Need a (submersible) optical switch (preferably not a float switch) to detect when the water level in the sump drops below a certain level it will automatically switch off the return pump. Anyone know any suppliers of these in NZL at a good price?

Try me!

Is the sump in the dark?

sump is in a room which has generally has the light switched off, so pretty much dark yes. why is that? optical light switch?

Many optical levle sensors are quite light sensitive. If it is in the dark you can get away with a cheap switch. If there is a lot of light at certain times you may need the non-light sensitive version. The non-light sensitive version cost about 4x as much...