A burst pipe, an uncontrollable leak, sewage backing up, no water at all, or a hot water system failure in winter. If water is spreading or services are down, call immediately and turn off the water at the toby (main shut-off valve).
Most towns keep 1-3 hour response windows for genuine emergencies; after-hours call-outs carry a surcharge. Calling early in the failure avoids overnight premiums.
You may replace washers and shower heads, but anything connecting to the pipework must legally be done or supervised by a PGDB-licensed plumber in New Zealand.
Expect a surcharge on top of the standard call-out for nights, weekends and public holidays. Ask for the total call-out fee on the phone before approving the visit.