The bridge was closed in mid-July, with the work scheduled to take about 14 weeks. Trouble stuck in early September, as rock was struck under the bridge and piles could not be driven to the depth specified by seismic requirements.
Waitaki District Council roading assets manager Gary Woock said on Friday it would cost an estimated $200,000 to fix the problem, as the northern abutment had to be redesigned.
An extra abutment and steel-encased piles below the existing road surface will solve the problem.
They will be connected to the bridge by concrete beams.
Mr Woock said the council still hoped to have the bridge reopened by mid-December.
The contractor had been asked to speed up the project.
Humber St, used by many as an alternative route to Oamaru's main street, Thames St, had been more heavily used since traffic lights were placed on Thames St, and some business owners in the area say the bridge closure has hit trade.
