Queenstown Lakes District Council infrastructure operations manager Simon Mason said the discharge from its Shotover wastewater treatment plant would start once work to remove debris and vegetation from the channel to the river was completed.
That was expected to take two or three working days, which means direct discharge will begin on Monday or Tuesday.
The council confirmed this week it would invoke emergency provisions in the Resource Management Act to begin discharging about 12,000cum of treated effluent into the river each day.
Cr Niki Gladding, who first revealed the council was considering the move after a public-excluded workshop with councillors last week, confirmed lobby group Aotearoa Water Action, of which she was a member, sent a letter to the council on Wednesday night.
Although its contents remained confidential, Ms Gladding said its purpose was to scrutinise whether the council had followed proper processes when it decided to invoke the emergency provisions.
The council had given itself 20 working days under the Local Government Official Information and Meetings Act to respond to a list of questions in the letter, by which time direct discharge would be well under way, she said.
Explaining the council’s decision to media this week, property and infrastructure general manager Tony Avery said the plant’s failing disposal field meant it had been breaching its resource consent for some time.
It had previously thought it did not have grounds under the RMA to invoke the provisions, but that had changed last week when Queenstown Airport Corporation asked for "urgent action" to address a heightened risk of bird strike caused by waterfowl attracted to the ponding in the disposal field.
The discharge would have no impact on downstream users, and would be "undetectable" in the Kawarau River, Mr Avery said.
However, the Shotover and Kawarau Rivers would be monitored to "ensure effects on the environment are understood and to enable action to be taken should anything unexpected be observed".
Once direct discharge starts, the council must lodge a retrospective resource consent application with the Otago Regional Council within 20 working days.
Asked what contingency plans were in place to respond to failures at the treatment plant once direct discharge began, Mr Mason said it would hold back wastewater in the adjacent oxidation ponds in the event of a "significant process disruption".
The ponds had "limited" storage capacity in such a scenario, and the "option to revert to the disposal field would also be available", he said.
The disposal field began failing in early 2021, about two years after it was commissioned, and issues with ponding were first reported by the council in a statement in August that year.
Mr Avery told media this week there was a "dramatic deterioration" in its performance about last August, causing continuous spilling of treated effluent into the Shotover Delta that raised "serious concerns about human health".
