It also showed that water sampling to determine the effects of the spill on the river did not begin until more than two hours after the overflow from a storage tank was discovered by a duty plant technician.
The council has not revealed how widespread the contamination was or how much the clean-up cost, because the matter is now before the court.
The council has not named the staff member involved or disclosed the person's position.
But it said that after an independent investigation, several improvements to the plant's processes - including training of staff - had been recommended.
A staff member said a planned $32 million upgrade at the Pukete wastewater treatment plant would result in "significant improvements".
The Pukete plant treats 45 million litres of wastewater daily.