KiwiRail is advising that repairs to the Tinwald viaduct could be six months away, and the road underneath it will be closed to traffic until then.

Signs and bollards preventing entry to the viaduct were installed on Melcombe Street and State Highway 1 on Tuesday.

An overhead impact beam on the viaduct was damaged a month ago by an excavator being towed on a trailer – the bridge has a low clearance for vehicles travelling underneath it and a history of frequent strikes.

KiwiRail engineers will undertake a full assessment of the structure with the viaduct closed to traffic. The current time estimate for repairs is 6-8 months, depending on the outcome of the detailed inspection.

Closing the viaduct to vehicles until repairs can be made will avoid further damage.

Pedestrians and cyclists can still use the footpath under the viaduct, while vehicles can cross the railway line at Compton Street, to the south.