One of 34 bridge piers was washed away in flood waters last Friday, causing the bridge to sag and prompting the closure of a 50-kilometre section of railway between Timaru and Ashburton.
KiwiRail said it was yet to be found.
Chief infrastructure officer André Lovatt said lower river levels had allowed crews to undertake a detailed assessment of the bridge and divert water away from the construction site.
"Our work now is focused on first making the bridge safe by ensuring the sagging spans do not collapse, which could fall into the river and damage the adjacent road bridge [on State Highway 1]. We will then install a temporary structure to replace the pier and allow trains to resume running over the bridge," he said.
A 250-tonne crane was on site and would begin driving large steel pipes into the riverbed from Friday, which would form part of the temporary structure.
"Today we are beginning to build the structure, known as a pigsty, which will allow us to get the sagging spans back into alignment," Lovatt said.
Once the bridge reopened, work would begin on a permanent pier replacement, Lovatt said.
Ashburton Mayor Neil Brown told RNZ's Morning Report earlier this week that the bridge outage was adding pressure to an already struggling roading system.
About 200 trucks per day were being used to transport freight between Ashburton and Timaru, he said.
That section of road was already struggling with congestion and had faced its own closure due to flood damage in the past, Brown said.
