Discolouration in Morrinsville Water Supply

We have received several complaints about discoloured water in different parts of Morrinsville.While the water looks unpleasant, it is treated to the New Zealand Drinking Water Standard and is safe to drink.

This discolouration is caused by higher than normal levels of manganese, which occurs naturally in our water supply as a result of the groundwater coming into contact with rocks or soils. The manganese levels in the storage dam tend to be higher in the warmer summer months.

The water treatment plant is having difficulties removing the manganese due to high levels of ammonia present in the water supply. Ammonia is also normally present in our water supply, but not usually at the level we are currently seeing. Our water team is working to resolve this issue and identify what is causing the high ammonia levels.

Please note that the increased level of manganese can cause staining to laundry or porcelain (e.g. your bath tub and toilets) - information on removing this staining is below.

Stain removal using Oxalic Acid

A solution of oxalic acid can help remove iron and manganese stains from clothing. It is available from businesses such as Bunnings or Mitre 10, generally in 500g packs, at a cost of around $18.

If the discoloured water has stained your washing, you should:

  • Ensure that your water supply is clear prior to using oxalic acid
  • Keep stained washing wet/damp
  • Add 75g of oxalic acid per litre of warm, clear water
  • This should be done in a large plastic bucket (NOT metal)
  • The stained clothing should be soaked in this solution for approximately five minutes
  • If a large quantity is stained it may be better to soak several smaller loads in succession so the solution is brought into contact with all parts of the affected clothing
  • Once the stains have been dissolved the clothing should be rinsed thoroughly in clear, cold water. The washing solution can be discarded by flushing down an outside drain

NOTE: Oxalic acid cannot be used as a measure to prevent the washing from being stained and therefore should never be added to the washing machine.


  • Correct usage and handling of this product should not result in any problems or health issues. While the acid is not corrosive like other acids, it is poisonous if swallowed. Consequently, should any spillage or splashing occur, ensure that it is wiped up immediately.
  • The oxalic acid must be kept out of reach of children. As an extra precaution, children should be kept out of the laundry while the cleaning operation is carried out. In the event of poisoning or suspected poisoning, immediately implement the first aid treatment outlined on the packaging.
  • Any Material Safety Data Sheet that may be provided by suppliers should be read and understood.