Sam if you are in Manukau and Adoge is in Epsom the water will be coming from two different places.
You will find all tap water has a high pH to prevent it from eroding copper pipes, but the kh and gh are low and it is easy to drop the pH. I'm in Pakuranga so will be on the same supply as you. I fill a 200L barrel from the tap 24 hours before a water change and dose it with Prime to remove the chlorine etc (and put a heater in over winter to warm it up). I have a pump in the bottom of the barrel and pump it into my tanks after a water change. I have found the pH drops below 8 while sitting over night, and in my 800L 6' which has a few big bits of wood I've had to put some coral rubble in the sump to prevent the pH dropping below 6. I also do 250-300L water changes in my two biggest tanks and top up the remaining 50-100L straight out of the hose with no prime or pre-heating, and my fish are fine (clown loaches, angels, uaru, etc).
How big are your tank/s and how much water do you usually change?