The roadway over the Fitzroy River in the Kimberley region is the state's only sealed roadway to the north.
Emergency Services Minister Stephen Dawson said there was significant damage to the bridge at Fitzroy Crossing and the Great Northern Highway.
Engineers and divers are expected to start inspecting the structure in the coming days to determine what can be salvaged and how much work will be required before the transport link can reopen.
"That will give us indication how many months or indeed years to build that bridge back up again," Mr Dawson told reporters on Tuesday.
"Early indications are that that work will take a significant amount of time."
He said any work was likely to start after the wet season ends.
State authorities are also looking at temporary transport alternatives, such as an old flood crossing across the river bed, and talking to the Australian Defence Force about building a temporary bridge.
In the meantime, road trains with freight for Kununurra and the Northern Territory have been diverted from southern WA in South Australia to travel up the Stuart Highway through central Australia.
A barge is also transporting essential goods from Perth Hedland to Broome, with authorities saying an airlift and boats might be the only way to reach some communities until the roadway is repaired.
It comes as a "massive" disaster relief operation hits full swing in the Fitzroy River catchment area, where many people remain isolated by floodwaters and damaged roads.
More than nine tonnes of food and 10 boxes of medical supplies have been flown in to flood-ravaged Fitzroy Crossing as more planes join the airlift.
"This is a massive operation under way to get essential supplies into that community and its surrounds," WA Fire and Emergency Services Commissioner Darren Klemm told reporters.
He said the airstrip was now dry enough to land larger aircraft and it would significantly increase capacity to resupply the town of about 1200.
"I want to reassure the people in the Kimberley region we are bringing a significant amount of food into the region to ensure we have adequate supplies," he said.
Authorities, with assistance from the Australian Defence Force, have also started the damage assessment and clean-up operation in the areas they can access.
It's hoped some people will be able to start returning to their homes but many areas remain inaccessible and others have no drinking water or power.
"This has been an unprecedented situation for WA. It's the worst flood of its kind in our history," Mr Dawson said.
"It is challenging and not something we've had to deal with before to this extent."
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