An anode rod is a magnesium or aluminium rod inside your hot water cylinder that corrodes deliberately to protect the tank lining from rust — it's called a 'sacrificial' anode. It typically lasts 5–8 years and should be inspected and replaced when worn to extend your cylinder's life significantly.
Inside every hot water cylinder, the metal tank lining is vulnerable to corrosion from minerals and oxygen in the water. The anode rod provides electrochemical protection: it sacrifices itself, corroding instead of the tank. When the rod is fully depleted, the tank itself begins to corrode — leading to rusty water and eventual tank failure.
An anode rod inspection and replacement in Christchurch costs $180–$350, including labour and the new rod. It's a straightforward job for a plumber. Spending $250 every 5–8 years to extend your cylinder's life by 5–10 years is excellent value compared to the cost of early replacement.