The SH83 twin bridges at Kurow are closed because of flows up to 1500cumecs in the river, while two trees washed down against the northern end of the SH1 bridge at Glenavy created their own problems.
The trees lodged against the bridge on Wednesday night and were hit by two trucks about 6am yesterday.
That caused damage to the concrete railing of the bridge, which was reduced to a single lane yesterday while a digger attempted to clear the trees.
The bridge was also closed for about 90 minutes after a four-car nose-to-tail crash yesterday morning.
Police highways patrol Sergeant Peter Muldrew, of Oamaru, said one of the trees extended on to the bridge when it was washed against a pier.
A diesel tanker carrying about 39,000 litres of fuel hit it, damaging the truck's door and the tanker. No fuel escaped.
Another truck following it also hit the tree, damaging its canopy.
Loss of the SH1 bridge, combined with the closure of the SH83 twin bridges would leave no road crossing of the river, traffic having to detour through Omarama-Tekapo-Twizel, a lengthy trip of about 280km.
The SH82 bridges at Kurow could remain closed for a week or more until the river recedes enough to allow the damage to be evaluated and repairs made.
Yesterday, the Waitaki River was flowing at about 1480cumecs at Kurow, and was expected to remain at those levels until at least Monday as Meridian Energy Ltd clears water from over-full hydro-electricity lakes in the upper catchment.
More rain is forecast for late tomorrow and Sunday in the upper Waitaki catchment, which could delay lowering the river. However, to provide some access between Kurow and Hakataramea, the New Zealand Transport Agency is allowing pedestrians and cyclists to use the bridge between 6am and 10pm.
The bridges are being manned 24 hours a day by contractors to ensure traffic does not cross.
Agency area manager Colin Hey said an inspection of the damaged pier on the north bridge could not be done until river levels dropped. The bed levels of the river next the bridge also needed to be checked to see if changes had occurred which could affect other parts of the structure.
"It is a frustrating situation," he said.
However, nothing could be done until the river dropped.
Once the results from the inspection were known, options to open the bridges could be looked at in more detail.
Among those were a Bailey bridge over the damaged pier or underpinning it.
"This is shaping up to be the longest closure of the bridges since 1994," he said.
