The new seal on the Dublin Street Bridge is in need of repair just four days after it was laid.
The bridge will be closed on Wednesday between 7pm and 10pm for the urgent repairs.
The bridge was resurfaced on Saturday when it was closed for the day but some of the chip has already lifted.
"The new surface applied to the bridge on Saturday was affected by the recent rain and did not adhere as expected," Whanganui District Council infrastructure manager Mark Hughes said.
Roading crews from Hawke's Bay and Taranaki have been brought in to support Whanganui crews with staff and equipment, and traffic management will be in place while the repair work is carried out.