Sylvan, I did say that, quote “you could fix the shower as suggested by Sylvan”. However this seems to be the standard plumbers “quick fix”, fix the symptom not the problem. Yes like Aussie, we Kiwi’s have tempering valves as part of our regulations (10 years here)and they are a great method of preventing scalding. The problem can be that if the supplies are not balanced some models cannot cope. A thermostatic mixing valve would be a better valve in that situation. You will also note that John L O said that only when the cold water was turned on elsewhere in the house the problem occurred.
There are too many unanswered questions from John L O to give a definitive answer:
1. Has the problem been from original installation?
2. If not, when did the problem start – was there some plumbing done?, some maintenace done on the water supply by the water authority?, Is there a blocked strainer?, or as Bungie suggested is there a galvanised water main?, is the reducing valve working properly?, is the plumbing done correctly?
Sylvan, for the sake of repeating myself, yes you can fix the shower but how about diagnosing that problem that is causing the shower temperature to fluctuate when another fixture is used.