The wooden remains - described as a find of national significance when unearthed - will be displayed inside the George St shopping complex once they have been treated. They have been soaking in a bath of polyethylene glycol, to strengthen the wood, following the causeway's discovery during the mall's construction in June last year.
About 60% of the causeway, made from manuka and kanuka, was previously found too rotten to save.
However, Southern Archaeology Ltd director Peter Petchey, whose firm was contracted by the council to treat the wood, said in a letter the process was expected to take "at least another 12 months".
His company's work was being overseen by wet-wood conservation expert Dilys Johns, of the University of Auckland, who said when contacted she was confident the wood would eventually be ready for display.
She had visited Dunedin on six occasions to examine the causeway since its discovery, and about 460 samples had been taken in that time.
Most of the remains were in "pretty poor" condition, but no worse than first feared, she said.
"A large proportion of it is very degraded, but some is not - some of it is not in too bad condition."
The treatment process had begun slowly, to protect the wooden remains from collapsing initially as the solution was introduced, but she expected the pace to pick up in the coming six months.
Asked if she was confident the causeway's remains would eventually be ready to be displayed, she said: "It will definitely be fine".
Mr Petchey said the treatment process involved soaking the timbers over an extended period in polyethylene glycol, which was "effectively a water-soluble wax".
The solution soaked into the timber's cell structure, providing support when the water was removed by drying, he said.
"Without this support, the timbers would crack and break up, as the cell structure has deteriorated during the 150 years that the timbers have spent in the waterlogged ground.
To attempt to shortcut the treatment would lead to premature deterioration of the timbers and the loss of the causeway," he said.
