"The erosion around the bridge is continuing to get worse. The Waiinu community has got an urgent problem with access," he said.
The road extension will cost an estimated $5.25 million. The New Zealand Transport Agency (NZTA) will pay most of that, with the South Taranaki council finding about $1.8m.
The council's detailed business case for the road extension was approved by the council and NZTA last year, but the cost has gone up since then.
Extra cost has arisen from upgraded intersections with State Highway 3 and Waiinu Beach Rd, a rail crossing upgrade, compensating private property owners and an archaeological assessment.
There are nine property owners affected, three with the road crossing their land and the remainder experiencing increased road use.
One of those with the road traversing their land, organic dairy farmer Diana Handley, has not agreed. The council might have to take 4.5ha of her land under the Public Works Act. All landowners will be compensated, Mr van der Merwe said.
Even with those cost increases the road will be cheaper than building a new bridge over the Waitotara River, a council meeting agenda said.
Mr Pearce said independent commissioner Christine Foster held a resource consent hearing for the road extension in Whanganui on August 7.
At the hearing Ms Handley and Christchurch-based landscape architect and planner Di Lucas presented a different route option, but the commissioner said she was unable to consider it.
The road's exact route depends on the result of the resource consent hearing, and Ms Foster's decision is due on August 25.
On Monday night South Taranaki District Council approved tendering the proposed road construction, subject to council approving the final tender.
