Your water heating system will determine if your water pressure is high or low. If you don't know how your water is heated, find your hot water system and identify it using this image.
If you have an electric water cylinder, you will need to find out whether it is Low pressure (30 - 120 kPa) or Mains pressure (120 - 1100 kPa). If this isn't written on your cylinder, click here for more ways to identify your pressure. Some electric hot water cylinders may also be outside, in a casing.