In January, flaking concrete was discovered beneath the 90-year-old structure, the second-oldest reinforced concrete bridge in New Zealand.
It had been scheduled for a full structural inspection this year, which was brought forward to February following the discovery of the concrete deterioration.
"A lot of the bridge is in outstandingly good condition. [However] some of the ... concrete [beneath the bridge] isn't of such high quality."
He said the reinforcing steel embedded in the concrete had rusted, causing the concrete to crack.
Contractors were removing the cracked concrete, removing the rust from rods and where the rust was significant, new rods were being installed.
Concrete was then being reapplied in some sections.
"It's only a couple of weeks' work, but they have got to have a break in the middle.
"We don't want to expose ... it all at once.
"We need to repair it section by section."
Traffic had not been affected by the repairs, nor had Shotover Jet's operation, which runs directly under the bridge on the Shotover River.
While the work was expected to be completed by the end of the month, Mr Marshall said the bridge would require a management process, including inspections on a regular basis, from now on.
"These ... are only the repairs where concrete has cracked as a result of rusting.
"There will be other pieces that rust ... over time.
"It now means we just need a management process of inspections - possibly every five years."
