Work on the contract, including construction of new piles, bridge deck, railings and road approaches for the bridge, which spans the Orari River, will start on Tuesday.
The single-lane Factory Rd bridge is the main access for one of Fonterra's largest plants, the Clandeboye Dairy Factory near Temuka.
Timaru District Council land transport manager Andrew Dixon said Waitaki District Council-owned Whitestone Contracting Ltd won the contract from five submitters, meeting stringent structural considerations and also offering the lowest price.
The 1960-era bridge will be widened to two lanes at a cost of $3.29 million, of which the New Zealand Transport Agency will provide $2.07 million under the funding assistance rate.
Mr Dixon said the project was expected to take 18 months.
The long construction period made allowances for times when certain work could not be performed - during fish spawning season and during peak traffic times, particularly during the spring-summer milking, he said.
Whitestone Contracting chief executive Glenn Campbell said it was the company's biggest current one-off project.
''It is [good news],'' he said.
''We were pretty pleased to get that [contract].''
Up to 1400 vehicles use Factory Rd each day during the summer.
Drivers using the bridge face delays because the 173m long structure is controlled by traffic signals.
NZTA regional planning and investment manager Bruce Richards said the work would greatly improve access along the road, particularly for milk tankers travelling to the dairy factory.
Clandeboye processes up to 13.2 million litres of milk a day - more than 40% of milk collected by Fonterra in the South Island.
The factory is serviced by New Zealand's largest liquid-load-carrying fleet of trucks.
The Factory Rd bridge project will be supervised by Opus Consultants, which also oversaw the building of the replacement bridges across the Waitaki River at Kurow.
