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An $18.5 million project by the New Zealand Transport Agency (NZTA) will replace the 132-year-old twin wooden single-lane bridges with two concrete two-lane bridges.
A special event is being planned to mark the start of construction.
A start on the project will begin to ease fears the old bridges could be so badly damaged in flooding they would close, fuelled by a lengthy closure in February 2011.
Part of the NZTA's $1 billion programme of investment in Canterbury's transport network from 2012-15, the bridges will take about 18 months to build.
The NZTA's southern regional director, Jim Harland, said the two ageing bridges had reached the end of their life.
''These single-lane bridges were opened in 1881 and, with a combined length of 762m, were one of the largest wooden structures of their kind in New Zealand.
''The predominantly timber structures have been deteriorating and are no longer suitable to serve the needs of the local community, being vulnerable to closure during peak river flows and unable to carry oversized agricultural machinery,'' he said.
The new bridges would provide a reliable alternative route to State Highway 1.
They would be built immediately downstream from the existing bridges and each would have a barrier-separated pathway for cyclists and pedestrians.
''Safety has been a strong focus with this project,'' Mr Harland said.
''This is an important route, not only for the local community to access services and for the distribution of supplies to the surrounding rural community, but also for the growing number of tourists and visitors heading inland through the Lindis Pass or the Mackenzie Country.''
Contractor McConnell Dowell Constructors Ltd will be on site from early next month to begin building the bridges, which are expected to open late next year.