The city's airport, harbour, Gilberd St boat ramp and parts of Castlecliff were at risk, the council infrastructure and special projects committee was told.
"Seafront Rd would again truly be Seafront Rd and the Castlecliff playground would be more of an aquatic park," manager Rowan McGregor reported.
Councillor Rob Vinsen went further: "The safety - the very existence of Castlecliff - is at risk."
Repairs of about $16.5m are needed. The council was unaware of the state of the structures until 2010, when it took back control from harbour lessees River City Port. A new report from engineers Tonkin and Taylor highlighted 15 items that required repair, including five items costing $750,000 that needed immediate attention.
The council said it felt the Horizons Regional Council ought to foot the bill.
Mr Little last night said major infrastructure work was needed at the port before its potential as a centre for excellence and economic development could be realised. "The council has told us that repairing and developing the port precinct is a very high priority in order to create a business hub that will attract industries and companies that have already indicated an interest in moving into the revitalised space.
"This redevelopment will create hundreds of jobs, and this project is part of a growing partnership between the council and iwi. It will help improve the health of the Whanganui River in recognition of its new legal personhood."
The first stage of the redevelopment was repairing the moles, he said.
"The moles are just one example of a crucial piece of regional infrastructure that has been allowed to decay under National. If they are left to fail, the mouth of the Whanganui River will potentially move and damage vital parts of the city, including the port, the airport, and up to 100 homes.
"Labour supports Whanganui's vision for its future and opportunities to develop its economy. Repairing the Whanganui River moles will protect and create jobs in Whanganui - it is an example of how Labour's regional development fund will help towns and cities prosper.
"It is time for central government to step up and play its part in investing in our regions. Labour will offer Whanganui council $3m to meet half the cost of structural work on the moles, with a matching contribution by the council."