Work on repairing the extensive washout alongside Anzac Pde next to the Whanganui River is now expected to start early next year.
Ross I'Anson, NZTA highway manager, has confirmed to the Chronicle that design work should be completed in December which means contractors should be on the job early in 2017.
The riverside walkway and edges of the road - which is part of SH4 - were damaged during the June floods last year when a part of the footpath near the Anzac Pde-Portal St intersection collapsed.
Then at the end of July another section slumped, taking out the footpath from Taylor St to the City Bridge.
At the time NZTA said engineering assessments after that second slump showed up a bigger - and costlier - problem than first thought. The final sum could reach $5 million.
"As we advised in August, we had identified potential issues with erosion of the river bank below the water level so we've been working with a geotechnical expert to come up with the right option for this area which has complex hydrology and river dynamics," Mr l'Anson said.
He said several options had been considered but based on advice from a geotechnical expert NZTA was now taking the best option through to a detailed design stage and that was expected to be completed in December.
"Having detailed designs and construction drawings will enable the contractor to start their preparations, including sourcing materials and subcontractors and we expect the contractors to start on site early 2017.
"It's important that we get the right design to repair this section of Anzac Pde and we appreciate the ongoing patience of the Whanganui community," he said.
NZTA will foot the bill for the project because it's part of the state highway network.
Mr l'Anson said they were considering using sheet piling, which uses interlocking sheets of steel to form a retaining wall. This retaining wall will allow contractors to clear the site of water, before building a permanent retaining structure.
This stretch of riverbank has been subjected to much more significant slumps before, including washouts from Georgetti Rd to Purua St, when both lanes of Anzac Pde disappeared, and then on Putki Dr downstream from the City Bridge.