The bridge over the braided Rangitata River in Canterbury is an important rail transport link for much of the South Island, and would normally be crossed by about 45 freight trains each week.
It was hit by flooding during storms that lashed much of the country on 12 April and washed out part of SH6 on the West Coast, between Franz Josef and Fox Glacier.
KiwiRail chief infrastructure officer André Lovatt said these repairs were temporary, and a design was being drawn up for a permanent solution.
"It has been a significant undertaking," Lovatt said. "First we had to divert the river away from where the pier was, then install a structure to ensure the bridge spans didn't sag further or collapse and damage the adjacent road bridge.
"We built a temporary support for the rail bridge, using a 250-tonne crane to push the four caissons [watertight chambers] deep into the river bed and install and weld cross bars into place. Finally we replaced the tracks across the two spans, which were warped when the bridge sagged."
A test train was run over the bridge on Anzac Day, with freight services expected to resume the next day.
Lovatt said "road-bridging options" had been provided to freight customers while the bridge was out of action, but the line being reopened meant there would be less heavy trucks moving between Timaru, Ashburton and Christchurch.
Initial work on the permanent repairs were expected to begin next week, and to take between 10 and 12 weeks.
They include driving new casings surrounding the bridge's support columns deeper into the river bed, and building a new steel and concrete pier.
Meanwhile, Lovatt said a search was on for the missing concrete pier, which could be buried in the river bed.
RNZ
