The Nanga Road Bridge in Lane Poole Reserve has been closed to vehicles and pedestrians effective immediately over major safety concerns.
The "unusual closure" comes after an investigation this week by Main Roads which found some of the bridge piers and piles, particularly in the centre, were deteriorating.
The closure will primarily affect those looking to travel south from Dwellingup over the Murray River.
In the drier months, there is an alternative crossing, Bob’s Crossing, but that is not currently open, and is unlikely to be for the rest of winter.
All access to the Nanga Brook area of Lane Poole Reserve will need to be via Waroona, a diversion that adds about ten minutes for those travelling from Pinjarra, and about 50 minutes for those coming from Dwellingup.

Shire of Murray President Cr David Bolt said while it will be frustrating for motorists and pedestrians, safety is their main priority.
“I recognise that this bridge closure is done with extremely short notice, but such is the condition of the bridge that Main Roads do not believe it is safe for vehicles to cross it, even today," he said.
“I am mindful of the excellent local businesses we have operating in the area, and we will be speaking to them about the potential impact of the bridge closure once we have a better understanding of the implications."
Main Roads is undertaking further assessments and its priority is to review design options to make the bridge accessible, and to instigate short- to medium-term solutions.
Park Road and River Road remain open, with Dwaarlindjirraap day use area, Baden Powell, Charlie’s Flat, Island Pool, Tony’s Bend and Yarragil campgrounds easily accessible from Dwellingup.
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