About $4.8 million of general maintenance work is under way on port assets around the harbour.
Port Otago does not expect there to be any disruption to shipping services, because the period, of up to 12 weeks, coincides with the off-season for many exports and should be completed before the new cruise-ship season begins in earnest.
Port chief executive Geoff Plunket said the two cranes on the main wharf would shift and join the third crane on the outer multipurpose berths while the crane rail system was relaid - two 330m strips.
The two cranes were able to turn the corner on the existing tracks to the multipurpose berth, which was a special requirement of their construction in China.
"Even if two vessels turn up at the same time, there would only be minimal delays," Mr Plunket said.
The two 1250-tonne cranes had 32 wheels apiece, each of which when fully under load exerted 68 tonnes of weight on the rails, Port Otago's civil engineer Andy Pullar said.
"We'll lift out each of the imbedded tracks, remove the eroded grout and the levelling plate.
"The [crane] movement and flexing has left cracks in the crane rail," he said.
Initially the old rails, laid in 1975, would be taken out over a two-month period, then the new rails would be installed, with completion set for November 7.
The new tracks include rebuilding of the grout in several layers and inclusion of a new continuous sole plate, with a rubberised mounting pad between it and the rail to dampen rail movement, and for thermal expansion.
The job has gone to Albert Smith Industries and its subcontractor Smith Crane and Construction, the former involved in building the multipurpose berth in the early 1990s.
Mr Pullar said while the main wharf was out of commission, about $160,000 would be spent on wharf concrete maintenance, and the company's suction dredge New Era would also dredge next to the berth, to a depth of about 13.5m.
The new tracks were expected to have a lifespan of 20-25 years, largely because they would carry heavier loads, and more often, than cranes had during the past 37 years, Mr Pullar said.
The newer, less worked, multi-purpose berth was showing no signs of stress and was not expected to require rail replacement for five to 10 years. Mr Pullar estimated the two 275m tracks could cost about $1.25 million.
A separate $3 million strengthening of Port Otago's 1960s oil wharf in the upper harbour was running almost to schedule, with the first of nine seabed foundations well under construction; and work on a second foundation had started last month.
The project on the oil wharf, used for offloading mainly petroleum products, is expected to be completed by late October.
