An Electricity Authority investigation has found more water was spilled from hydro dams than needed for 15 days last December, which ultimately led to higher power prices for customers.
The preliminary findings are the result of a complaint of undesirable trading made by a group of eight small electricity providers against Meridian and Contact Energy.
The Electricity Authority later widened the probe to include Genesis Energy.
The Authority has confirmed that undesirable trading occurred between the 3rd and 18th of December, but it is yet to decide what action to take.
The Authority is now investigating whether the actions were also a breach of the high standards of trading conduct code, provisions which carry pecuniary penalties.
RNZ's Andrew McCrae is covering the story.