Wyley's Bridge, "the gateway to Mangamahu", needs urgent repairs so trucks can keep using it until a new bridge is built, the Wanganui Rural Community Board heard on Wednesday.
A report from Opus consultants found three of the bridge's timber transoms were rotting, and needed replacement.
That replacement should still allow loaded truck and trailer units to use the bridge for the two years before a new one can be built, Wanganui District Council's infrastructure manager Mark Hughes said.
The single-lane timber bridge was built in 1957. It crosses the Whangaehu River and opens up access to Mangamahu and the Whangaehu Valley beyond. The alternative route from Wanganui traverses the tortuous Burma Hill via the Parapara and Field's Track.
Without the bridge "you might as well just give up the whole valley," councillor Nicki Higgie said.
It was built of inferior timber, and parts are wearing out. It now has a speed limit of 15km/h, to prolong its life.
If urgent repair is not done the bridge will have to have a weight limit that excludes fully loaded stock and logging trucks.
Mangamahu farmer Andy Collins said walking stock across the bridge would be impractical.
He estimated that $15 million to $20 million of product came out of the valley every year or more than 100,000 stock units, 500 tonnes of wool and 600 tonnes of kiwifruit. By 2020 an additional million tonnes of timber is predicted.
There are heavy loads to go into the valley too, such as superphosphate fertiliser, maize for stock feed and metal for roads.
Losing the bridge would cut the valley off and reduce property values. An alternative road that avoided a river crossing would cost up to $10 million.
Replacing the transoms would cost an estimated $150,000, Mr Hughes said.
The New Zealand Transport Agency (NZTA) would pay 62 per cent, Rangitikei District Council would pay a third of the remainder, with Wanganui paying the balance. Mr Collins wanted immediate action taken toward replacing the bridge, which could take two years and get a 72 per cent subsidy from NZTA.
He said Mangamahu residents would be asked at their July 1 meeting whether they were willing to put the $214,000 they have raised in a special rate toward the replacement.
The board recommended to Wanganui District Council that the transoms be replaced and moves toward getting a new bridge start immediately.