After Neighbourly.co.nz posting of:
Cashmere News Diary: 16 June 2016 – Water quality cover-up? cashmere.org.nz/news/Cashmere-News-Diary-16Jun2016.html
Rebecca Warwick from Cracroft commented:
Our element also needed replacing shortly after the earthquake (our house was only 7 months old). Haven’t had a problem since.
Cylinder make and type: HJ Cooper, mains pressure, solar ready.
Karen Degen’s and Rebecca’s replies reinforce each other and offer a clue. The sudden spike of cylinder corrosion post-earthquake could have been due to the short-term addition of chlorine as a steriliser to the water supply. Did this make it slightly acidic, causing the corrosion, at that time? It doesn’t explain why it’s just a Cashmere-area problem though, nor why it is still continuing (when the chlorination has long ceased.)