I have another one of these...so far i have checked the following and the problem is still present.

I have checked and secured the main incoming earth...even sanded the lugs and washes to make better contact.

Done a loop impedance test at the meter box...main DB and sub DB,s... 1.3 ohms ( Z= V/2 I) 238 volts/ 2 x 80 amps = 1.49 ohms max

Installed and replaced bonding straps and re terminated the earths on all the geysers.

Done insulation resistance tests on the geysers (all 5 of them)

Gone around the building and secured and added bonding where it had broken off or found loose connections.

Checked the voltage L-N = 238v ... L-E = 237v ... N-E = 0.5 - 3.8 volts (now this concerns me the way it moves up and down)

Did earth continuity tests from the earth bar to the taps o.1 ohms and lower if i check on the copper pipes.

If i do a potential difference test between the taps and outlet/drain (all PVC pipes with metal covers in the shower) ... the voltage ranges from 0.5 volts to 5 volts