Work is under way on repairs and reinstatement of the damaged section of the retaining wall at Anzac Parade next to the City Bridge.
New Zealand Transport Agency (NZTA) regional transport systems manager Ross I'Anson said the project would return the state highway to its full and unrestricted lane width, including reinstating the cycleway and footpath.
The cycleway and footpath was closed after the June 2015 flooding when the underslip resulted in "significant loss to the road shoulder" which is now being repaired.
"These works have taken some time to commission as the site is very complex and we have had to undertake extensive investigation and design works," said Mr I'Anson.
"We're pleased the repair work is now able to get under way."
Whanganui Mayor Hamish McDouall said NZTA deserved credit for starting the work at the proposed time.
"They said they would start work in October and the work was under way by the end of the month.
"It will be great for people in Whanganui East who rely on the footpath to feel like they are connected to the city again.
"I know people who rely on mobility scooters and it has been really hard for them."
Mr McDouall said it is to be hoped that the "weather will behave itself" until all repair work was completed.
Enabling works are under way now, and Mr I'Anson said Stage one work would begin on November 13.
"Stage one will involve the relocation of existing underground services onto the southbound lane, followed by the creation of a vehicle turning bay and an access track to provide for long term scour protection."
Stage one is expected to be completed in February next year and stage two is planned to begin in March 2018.
Stage two will include construction and reinstatement of the retaining walls at both the northern and southern ends, construction of the adjoining cycleway and footpath, followed by resurfacing of the damaged seal within the works site.
It is estimated that the $7m project will be completed by December next year.