Tairawhiti Roads general manager Dave Hadfield said repairs would cost about $100,000 and were needed to replace corroded concrete struts that carry utility services under the bridge.
Mr Hadfield said work would take about three weeks to complete and he urged river users to be aware that scaffolding will be installed under the bridge. That will reduce the clearance, particularly at high tide.
Work will start on the city side and move to the other side of the river.
Pedestrians using the footpath under the bridge and recreational river users could be affected.
The footpath will be closed at times while the scaffolding is put up, but contractors will keep it open as much as they can.