Mitchell's Causeway in the Blue Mountains was closed on Sunday after significant cracking and movement was detected in the road's substructure.
Further investigation has revealed that extensive testing and repairs are needed to render the section of the Great Western Highway safe for traffic.
"I will be honest and upfront to the people of the Blue Mountains and the Central West: this closure is expected to remain in place for at least three months," Roads and Regional Transport Minister Jenny Aitchison said in a statement on Friday.
"It is incredibly serious."
Transport for NSW advises at least two weeks of specialist geotechnical testing and 3D imaging must be carried out with no passing traffic to understand the full extent of the damage.
Even under the most optimistic scenario, any remediation required to make the road safe for vehicles would take at least a further two months, the government said.
"This is a complex site, with a unique heritage structure on a mountain pass," transport deputy secretary Matt Fuller said.
"The level of movement detected means we need time without traffic to fully understand the risks and determine the safest solution."
The causeway carries heavy trucks and other traffic down the slopes of Mount Victoria to Lithgow, the gateway to NSW's Central West region.
It's the main route between Sydney and Bathurst, which is due to host an NRL fixture between the Cronulla Sharks and Penrith Panthers on Saturday night.
Trains between Bathurst and Katoomba will be free from Saturday with extra services between Bathurst and Mount Victoria from Sunday.
An alternative road route via Darling Causeway is adding about 25 minutes to crossing the Blue Mountains.
"I know this closure is causing real hardship for families, workers, school communities, freight operators and local businesses, and I am deeply sorry for that disruption," Ms Aitchison said.
"But there will be no shortcuts on safety – we will not risk lives."
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