The breach in a Taieri River floodbank that kept the Otago Regional Council and farmers busy when the river rose with heavy rain at the weekend was caused by an old steel pipe, of unknown origin.
On Sunday, excavators spread truckloads of rock below the floodbank to stem the flow of a breach near Dunedin Airport.
At the time the council could not say how the breach occurred.
Yesterday, council engineering manager Michelle Mifflin said the cause of the breach near Otokia had been identified after working closely with the landowner, site monitoring and inspections, and receiving geotechnical advice.
During peak levels of flow in the river at the weekend, the water found its way through the pipe, which allowed water through the floodbank, Ms Mifflin said.
Council staff were working yesterday afternoon on backfilling the hole formed around the end of the pipe and capping the pipe as a temporary measure until after an investigation into the damage done to the floodbank was complete, she said.
Further investigations were also under way to establish a long-term solution, Ms Mifflin said.
These investigations included hydraulic modelling, which would help the council assess the need for risk mitigation ahead of a permanent solution.
Between staff and contractors, the bank had been continuously monitored since the issue was first identified on Sunday.
The situation was stable, she said.
