KiwiRail public affairs manager Kevin Ramshaw said bridges 217 and 218 were built in 1927 from steel and timber.
Because of their age and the harsh marine environment in which they were situated, the 13m bridges were subject to a 25kmh speed restriction.
Once they were replaced, the speed restriction could be lifted to 80kmh.
The new bridges are being designed to enable a cycle path to be added by the New Zealand Transport Agency, which has approval to spend $6.8 million completing the cycleway between Dunedin and Port Chalmers.
Mr Ramshaw said the bridges were already used "extensively" by cyclists and KiwiRail had expressed concerns about cyclists trespassing on them.
The bridge work should be completed by the end of the year and Mr Ramshaw said the final cost would be determined by the tender prices.
