The main road on the West Coast could be closed for more than two days following flooding near Harihari.
Transport agency spokesperson Mark Pinner said a temporary bridge will be transported from Christchurch to restore access if it is needed.
Mr Pinner says there are no alternative routes in that region between the West Coast, Wanaka and Queenstown, and drivers are being diverted east of the Southern Alps.
Further north, a slip has closed State Highway Six between Inangahua and Westport.
Flooding has also closed Arthur's Pass between Bealey and Jacksons.
The closure of State Highway 73 means the nearest detours are lengthy ones, via Wanaka or the Lewis Pass.
MetService said the flooding is the result of about 350 millimetres of rain falling in the area over the past two days.
Communications company, Chorus, said the Wanganui River bridge washout has damaged cabling.
It said any repair work will not be able to be carried out until Thursday morning when it plans to deploy a temporary fibre cable over the river.
On Wednesday evening Chorus said that residents connected to the Fox Glacier and Franz Joseph exchanges should still be able to make calls locally, and a technician with a satellite phone was being sent to monitor emergency calls from those exchanges.
It said Haast and Harihari residents would be without phone services, as would some other rural areas.
And in Canterbury police are advising the public to avoid the Rakaia Gorge.
They said the Rakaia River is extremely dangerous and people should evacuate the Rakaia huts and low lying areas until the river subsides.