"When that all combines, we end up with areas with pipework that do not have the capacity to manage it."
The result in the 2015 flood was manhole covers that "popped out" in South Dunedin from the pressure.
Mr Dyer said work in Kaikorai Valley to renew the pipes was to have started about now, but only one tender had been received, and it was declined.
The council would "go back to the drawing board", but he hoped work could start early next year.
He said while the damaged network in Kaikorai Valley was an issue for South Dunedin, work had been done to mitigate problems there.
The Kaikorai Valley work, once completed, would alleviate the pressure, but was not a solution for what he said was a "big picture" issue.
The issue of the delayed drainage contract came up at a recent council meeting.
Cr Lee Vandervis asked about what he described as "the backlog of drainage spending for the city".
That lack of spending "affects people terribly" when there was flash flooding.
Mr Dyer said the "Kaikorai Valley three" project was the renewal of wastewater pipes in the Nairn St area.
Ultimately, the full length of the pipe along Kaikorai Valley would have to be renewed, but the initial work was to have been from Bishopscourt through the Kaikorai Common reserve to School St.
The work would eliminate "inflow and infiltration", groundwater that flowed into old, cracked and warped pipes.
He said the timing of the Kaikorai Valley three project would depend on how it was carried out, but it was expected to take up to two years.
"That's one of the reasons we've decided to go back to the drawing board, and think about how we can do this smarter and how we can deliver it with fewer effects on the community."
He put the difficulty getting a contractor to do the work down to a busy construction sector.
The University of Otago was undertaking plenty of work as it spent hundreds of millions of dollars upgrading the campus.
As well, the council water and waste and transport departments were "ramping up" their workloads.
Mr Dyer said this year's water and waste budget was about $16 million, climbing to a peak of $22 million in about 2020, "climbing in the order of 20 per cent to 30 per cent".
"If we're doing 20 per cent to 30 per cent more work, we're theoretically going to need 20 per cent to 30 per cent more resourcing from our contractors to deliver that.
"That's one of the challenges we face. Contractors don't like to commit to employing a whole lot of people if they can't see a sustainable workload."
Mr Dyer said the council also needed to consider "how we appeal as a client to the construction market".
It had sent letters to about 500 householders explaining the situation.
He hoped the work would go to tender late this year or early next, and construction would begin early next year.
A culvert in School St that had been overwhelmed on occasions in the past few years, flooding properties, would be fixed in mid-October despite the delays.
In 2015 the council bought and demolished a house in the street that had been at the centre of a long-running sewage contamination problem.
Mr Dyer said increasing the size of the culvert would fix that problem.
That work was "time critical" and would be put to the market as a smaller package.
"The other work will benefit the whole wastewater network down to South Dunedin, and storm events when we get high groundwater and that groundwater starts to flow into the wastewater network."
