The most likely reason is that we alternate actual and estimated meter readings.
The chart below shows an example of this scenario. The first high bill due to a leak is in May. The leak is fixed sometime in June or July, but there is a second high bill in July.
The estimate in June is significantly lower as it is based on the last two normal actual reads (January and March).
The July bill is still higher than usual because it has recorded the rest of the leak between receiving the May bill and when it was fixed (sometime between the June and July bills).

Click to enlarge (PDF)
If you think this situation applies to you, please contact us so we can consider a further allowance. Phone: (09) 442 2222, press 2.