I think(!) the NZ system is similar to those used in most European countries - 230/400V wye with grounded center point. The center is always grounded at the transformer, sometimes also locally (as per the ground-neutral bond in US main panels though not necessarily at that specific location), essentially making the difference between TT system (neutral not bonded locally) and TN (neutral bonded locally in modern systems). Several countries use different terms for TN systems like MEN (multiple earthed neutral) or PME (protective multiple earthing).
You can also find old systems where the neutral is not bonded locally but all metal surfaces are bonded to the neutral (PEN) bus bar. In this case transposing the phase and neutral would not be noticed in a single phase supply but cause all bonded surfaces to become hot. This occasionally happens in Germany when morons start working on older systems, particularly systems with a neutral-ground bond in each receptacle box and no color coding on the wires at all.
I agree though, the 180V to the water are likely a phase to ground voltage using the water as a (poor) ground.
