Responding to criticism from retired engineer Bruce Hendry about a lack of action, a council spokesperson said work was under way to achieve stormwater upgrades that would "assist in providing resilience" but had limitations.
"It’s not necessarily going to be the best, cheapest or only approach to be adopted, nor the most successful or affordable standalone approach," the spokesperson said.
The Dunedin City Council had focused on immediate works needed, while planning and consulting over the "much more extensive and expensive work to come" once solutions were determined through the South Dunedin Future programme.
He said he was the "last surviving member" of the Dunedin Drainage and Sewerage Board staff employed on a stormwater upgrade done in the 1960s.
His concerns centred around the limited capacity of the system to flow and pump out water in the face of "global warming with unprecedented rainfall that could come any time in the near future".
There had been "many reports but no action", he said.
He described the concept of lakes or open channels for the area as an "absolute pipe dream" because of runoff from streets being polluted and any standing water also needing to be pumped.
Responding to Mr Hendry’s concerns, the council spokesperson said work was under way to redirect some water.
Some of the stormwater flowing north from mud tanks in Forbury Rd into South Dunedin’s pipe network - to be discharged at the harbour through the Portobello Rd pump station - was instead better diverted south to the Pacific Ocean through a concrete channel, the Forbury aqueduct, that discharges at Second Beach.
Work to redirect water would finish in October and part of the aqueduct had been strengthened to "reduce the risk of future failure", the council spokesperson said.
This would "provide improvements to management of surface flows" in South Dunedin.
Work was also under way to enable stormwater to flow better through the pipe system to the Portobello Rd pump station.
The council had been aware for 14 years of several "hydraulic bottlenecks" upstream - a 2011 report from Opus had flagged this four years before the catastrophic 2015 flood.
An investigation in 2020 into bottleneck removal by WSP (formerly Opus), Stantec and Beca had determined "hydraulic interactions were complex and not readily addressed through addition of new pipelines".
The council spokesperson said a modelling tool to "ensure the hydraulic interactions are completely understood" was now being prioritised, leading to a plan for work by mid next year.
One bottleneck risk had been addressed. Since the 2015 flood, a new filtering screen upstream of the Portobello Rd pump station had been installed that was easier to clean during high flows of water, plus more alarms that warned if a filter was becoming blocked.
Stormwater pipes and mud tanks were also being inspected.
One impact of flooding in the past had been stormwater entering foul water pipes from roof pipes that were wrongly routed and because of cracks or poor joints in foul water pipes, causing them to overflow with sewage.
In Surrey St, residents have in the past suffered foul water sloshing back on to properties. Between 2015 and 2020 the council installed "reflux" valves on the street to prevent this.
The council said work continued through the South Dunedin Future programme and Three Waters integrated system planning to look at "measures to mitigate inflow of stormwater into foul sewers".
Across the city, $80 million was being spent in the 2024-25 annual plan on water infrastructure, an increase of $32m compared with figures in the most recent 10-year plan.
Within South Dunedin, the council said "all potential options for flood alleviation and climate adaptation for the wider South Dunedin area are being explored ... including on the future use of Forbury Park".
Mr Hendry welcomed the Forbury aqueduct upgrade but said measures such as reflux valves were of small remedial value and would not fix the bigger problem.
He had "ongoing concerns and watches with interest to see what is done".
