No — you cannot legally replace your own hot water cylinder in New Zealand. Under the Plumbers, Gasfitters and Drainlayers Act 2006, all sanitary plumbing work must be done by a licensed plumber. DIY installation is illegal, voids your insurance and warranty, and can create dangerous safety hazards.
NZ law classifies hot water cylinder installation as 'sanitary plumbing work' — a category requiring a licensed tradesperson. This isn't bureaucracy for its own sake: hot water cylinders operate under pressure and at high temperatures. Incorrect installation can result in over-pressure failure, scalding, flooding, or in extreme cases, catastrophic tank failure. The legal requirement exists to protect homeowners and occupants.
While you can't do the installation work, you can:
If unlicensed work is discovered (e.g., during a home sale inspection, or when you make an insurance claim), you'll need to have the work removed and redone by a licensed plumber. You'll pay twice — plus potential fines. The insurance claim for any water damage from DIY work will likely be declined.