The Electricity Authority defines an MDC as 'a domestic consumer who is dependent on mains electricity for critical medical support, such that loss of electricity may result in loss of life or serious harm".
This includes medical equipment and other electrical equipment needed to support medical treatment (for example, a microwave that's used to heat fluids for renal dialysis).
You can read the Electricity Authority's advice for MDCs here.
All medically dependent customers should have an emergency plan in case the power goes down.
ERANZ has prepared an emergency plan checklist for MDCs, which you can download here.