Engineers working to fix a bridge that was damaged when a truck crashed this morning, seriously injuring the driver, have accidentally cut a fibre optic cable causing communications outages in Marlborough.

Police are asking motorists to avoid State Highway 63 in the Wairau Valley. The crash happened on Wye Bridge, 50km south-west of Renwick, about 5.45am.

"While engineers have been attempting to fix the bridge, a fibre-optic cable has been accidentally cut causing communications outages.

"This may have caused banking and EFTPOS outages in the Marlborough area.

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The bridge is expected to be closed for most of the day. The driver was airlifted to hospital.

Motorists are asked to avoid the bridge and the alternative route is via a ford bypass. This will only be passable if weather conditions are favourable.