fearandloathing: Sonoff devices are not certified for use in NZ as far as I can tell? Plus not burying non certified electrical devices in the wall cavity, and equally the complications of insurance, should my house burn down.

All good points but...

Thousands and thousands of these things are being used all around the world anyway. Why are they not building them with a port to sense the existing wall switch? The additional marginal cost is one PCB screw terminal. It makes no difference to the electrical safety either way.