The collapsed bridge at Fitzroy Crossing is crucial to the Great Northern Highway transport route, which links the north and south of the state, and will take years to rebuild.
Tonnes of food and boxes of medical supplies have been flown into the disaster-ravaged region, where many people are isolated by floodwaters and damaged roads.
Acting WA Transport Minister John Carey said a preliminary damage assessment on Wednesday showed about half the bridge had been swept away following the once-in-a-century floods.
He told reporters one option for reopening the critical road-freight link was building a temporary structure to replace the destroyed sections.
Main Roads WA said this could involve the Australian Defence Force building a Bailey bridge, a portable pre-fabricated structure, to allow lighter vehicles to use the highway.
A spokesman told AAP preliminary discussions were underway about the temporary option, although it wouldn't allow heavy road trains to make the crucial journey with supplies.
A low crossing on the Fitzroy River bed further north will need to be built to accommodate heavy trucks and enable construction of a permanent bridge at Fitzroy Crossing.
It will be able to take triple road trains, which can weigh up to 170 tonnes, but construction will take four months. A suitable site won't be chosen until flood levels drop.
In the meantime, road trains with freight for Kununurra and the Northern Territory have been diverted into South Australia to travel up the Stuart Highway through central Australia.
A barge is also transporting essential goods from Port Hedland to Broome, with authorities previously saying an airlift and boats may be the only way to reach some communities with fresh produce until damaged roads are repaired.
Design and construction of a final, replacement bridge at Fitzroy Crossing will take up to two years, the department's spokesman said.
Further assessments will need to be carried out once the water level drops further to understand why the bridge piers that held up the structure failed.
Main Roads had hoped to send divers in on Wednesday, but the spokesman said this was postponed due to the risk from fast-flowing waters, debris and crocodiles.
Mr Carey said 60 workers had also been deployed to repair accessible sections of the highway, which remains closed for 700km between Broome and Halls Creek and partially open to high-clearance vehicles in other areas.
Record major flooding continues downstream from Fitzroy Crossing at Willare amid fears the bridge and highway there could also be damaged.
Derby also remains isolated and minor flooding continues from Fitzroy Crossing to Noonkanbah, although water levels are receding rapidly.
Meanwhile, authorities continue to assess flood damage to about 130 government-owned rental homes in Fitzroy Crossing.
Mr Carey said plans for carrying out repairs and rebuilding were underway but it would take a significant amount of time to carry out the work.
"We are facing a heated construction market. This is northern Australia. It is far more difficult to get tradies here," he said.
The flood damage was caused by ex-tropical cyclone Ellie as the storm system circled the remote town, dropping unprecedented rain last week.
The weather system has since weakened and moved over the Northern Territory and Queensland, bringing rain and thunderstorms.
