420,000 litres of partially treated effluent thought to have spilled into Queenstown swamp

Hundreds of thousands of litres of partially treated human effluent spilled into a Queenstown swamp before it was stopped.

The wastewater entered the natural swamp after spilling over the top of an oxidation pond at the Shotover Wastewater Treatment Plant on Monday.

Queenstown Lakes District Council estimated about 420,000 litres of partially treated effluent passed into the swamp - an Olympic-size swimming pools usually contains about 2,500,000 litres.

Property and Infrastructure acting general manager Simon Mason said the material in this pond has typically been through 80 percent of the normal treatment process.

"We are still working to confirm exactly why and how this situation occurred, but the weekend's heavy rain and a control system issue are likely to be the key issues," Mason said.

The council was able to bring the pump back online, allowing them to pump effluent back onto the dispersal field, he said.

"We remain confident that the spill was fully contained within the swamp with nothing entering the (Shotover) river.

"Contractors have cleaned and sanitised the part of the road affected. The swamp has soaked up the spill with no further action required."

People could continue to use walking tracks in the area, but Mason said they should stick to the track and avoid the swamp at the delta, which has signs in place.