Original plans were for new single-lane bridges, but after a cost analysis and lobbying from the community, the agency has decided to build two-lane bridges.
Construction of the bridges on State Highway 82, estimated at this stage to cost about $15 million, could start in 2012 and be completed by 2014 if funding was available, NZTA South Canterbury area manager Colin Hey said yesterday.
In the meantime, the 129-year-old wooden bridges will still undergo regular maintenance, with a week's work planned next month.
The bridges will be closed from February 8 until February 12 between 7am and 7pm, but open for 10 minutes every hour to allow traffic through.
Emergency vehicles will be allowed across straight away.
Mr Hey said the work was to replace and stabilise decking which had become loose.
The work was regular maintenance, not because of recent high flows in the river.
"That sort of regular maintenance is not unusual for a wooden bridge," he said.
Mr Hey said the decision to build two-lane bridges was made late last year.
A tender would be advertised within the next two months calling for designs for the new bridges, which would be next to but downstream of the existing structures to use existing approaches and protection work.
There would be an increased cost for two lanes, but Mr Hey said it was not substantial.
"It's really just about as economical to do two lanes as it is one. It will also meet the standards we are aiming for on state highways."
The old bridges would be demolished and already there had been expressions of interest in the timber.
The New Zealand Historic Places Trust had been consulted because of the historic nature of the old bridges.
The twin bridges were originally built for both road and rail traffic, opening on November 7, 1881.
