Western Bay Council utilities manager Kelvin Hill said the work on the old timber railway bridge had been initiated by the asset owners KiwiRail.
"The work is important to ensure the existing railway line remains operational both in the short and longer term, given the significant importance for transportation of materials and goods within the region."
Hill said the pathway ran beneath the bridge by agreement with landowners KiwiRail.
He said there was a significant risk to the general public from construction activities around the bridge area while work was being undertaken. This included high numbers of construction vehicles on the access road to the council's animal pound and treatment plant.
"For this reason the public has been restricted from this area and hence the closure of the walkway until the new concrete bridge is completed. The public walkway will then be reopened. Additionally, overhead permanent safety protection measures will be installed to protect the public using this track.''
Hill acknowledged earlier notification of the closure could have been provided.
"We regret the inconvenience to users and acknowledge we could have done a better job in communicating the closure to cycleway [and pathway] users earlier."
He said there would be positives from the work, with KiwiRail picking up the tab for renewing the walkway under the bridge and paying for the required safety fencing.
KiwiRail senior communications adviser Simon Kilroy said the council had been kept fully briefed on the project, including site meetings beginning six months ago, as well as advice in May that KiwiRail would require the pathway's closure.
"The council, which is responsible for the cycleway asset, approved the closure plan," he said.
The bridge was reaching the end of its useful life and was being replaced as part of a nationwide scheduled replacement programme.
Grant Dally, a member of Te Ara Kahikatea Pathway Trust, said the trust knew the closure was coming.
"The last we had heard was that it was going to start in August, but we had not had official word from anyone."
The walkway was officially opened in April 2018.
"It was always coming. It's unfortunate that it happened after we opened the walkway."