Stray voltage. There is either a poorly sized neutral, or a poor grounding bond somewhere. If you are handy with power distribution, you can investigate yourself; if it were me, I would need an electrician, and would probably start at the newly installed light...
68,000litre, IG vinyl, LorentzPS 600 Solar Pump & 1/2 HP Tristar, Sandfilter, Aquarite T-15 SWG