Rusty or brown hot water in Christchurch most commonly indicates internal corrosion inside your hot water cylinder. It can also be caused by corroded pipes or a depleted sacrificial anode rod. If only the hot water is discoloured (not cold), the problem is in your hot water system.
Inside every hot water cylinder is a magnesium or aluminium anode rod that corrodes deliberately to protect the tank lining from rust. When fully depleted, the tank itself starts to corrode — causing rusty water. Anode rods last 5–8 years; regular replacement can extend cylinder life by 5+ years.
Rusty water indicates your cylinder is corroding and could fail suddenly. Have it inspected promptly. A cylinder showing internal rust is often only months from tank failure.