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The bridge will be closed between 9am and 5pm on July 4 and 5 so workers can safely put up scaffolding.
There may be other closures during the repair project. The bridge will be open at night.
Waka Kotahi NZ Transport Agency has set a 25-minute, 32km detour along Camphill and Kane Rds.
Waka Kotahi maintenance contract manager Peter Standring said the pier damage was identified during inspections earlier this year and the repairs were critical.
The bridge is 107 years old.
Work would include concrete repairs and extra strengthening.
"It is always challenging to undertake work on this historic, single-lane bridge safely, while minimising the impact on our road users. During the repair and strengthening works, Aspiring Highways will also investigate a new deck design," Mr Standring said.
Overweight vehicles may need to use alternate routes or travel at night, he said.
Arrangements have been made to accommodate school buses and emergency vehicles.