The new $25 million Arahura road-rail Bridge on the West Coast officially opens this Friday 11 December 2009 - several months earlier than planned.

One of the largest bridge projects on the New Zealand rail network in recent times, the new structure replaces a wooden, single-lane road-rail bridge which for more than 120 years provided a crucial link over the Arahura River for both State Highway 6 and the Hokitika Branch Railway line.

KiwiRail Chief Executive Jim Quinn said the early completion was a testament to those involved as replacing the historic bridge had been complex.

"This hasn't been a typical rail bridge project - instead we've created a single-track railway, a two-lane road and a pedestrian / cycle way all on the same foundations," Jim Quinn said.

"The old bridge was nearing the end of its life so securing this vital transport link for the future was important for the West Coast. This supports KiwiRail's ongoing programme to remove timber from its rail bridges. It is all about improving the reliability and efficiency of the network to meet the demands of rail's customers."

NZ Transport Agency Regional Director Mark Yaxley said around 3,500 vehicles use the bridge each day which meant one of the main requirements of this project was to keep State Highway 6 operational while the new bridge was built.

"As well as keeping the road open while the bridge was built around it, construction was also staged to manage both the whitebait and dairy dry seasons," Mark Yaxley said.

Mr Yaxley added that having the bridge open ahead of plan is a sign of how successfully the different demands have been managed.

"Both NZTA and KiwiRail feel that the help and co-operation of the local community, including owners of the riverbed, Mawhera Incorporation has been central to the success of this project."

Physical work on the new bridge began in mid 2008 with the installation of foundations on either side of the original structure. Traffic was diverted onto one road lane of the new bridge early in 2009 before work began during the dairy dry season when trains do not use the line, to decommission the old bridge and replace the rail line.

New guide banks were formed as part of the bridge project to reduce flood risks. Further safety improvements were achieved by raising the bridge to allow bigger floods underneath.

A single span from the original bridge has been retained to be used in a small heritage park nearby, which will be completed and opened early in 2010.