Those in the Batlow and Wondalga areas have been urged to get out now because when the Dunns Road fire hits, the town can't be saved.
The RFS declared the Dunns Road fire is expected to hit Batlow on Friday afternoon and when it does, the township will "will not be defendable".
The huge fire will meet even worse fire conditions with extreme fire danger forecast for Saturday.
"If you are in this area, particularly in the general area from Batlow north to Wondalga and west of Blowering Dam, you need to leave before tomorrow," the RFS outlined in the leave zone notice, issued on Thursday.
"Fire is forecast to impact the township of Batlow tomorrow afternoon.
The township will not be defendable.NSW RFS
"Embers from the fire may land in pine plantation adjacent to the township near Old Tumbarumba Road. It is therefore advised to leave now."
An evacuation centre has been opened at the Equex Centre in Wagga, which should relieve pressure on the current evacuation centre in Tumut.
NSW premier Gladys Berejiklian declared a state of emergency on Thursday afternoon. It will come into effect on Friday morning.
"All our personnel, all our agencies know that from tomorrow they will be subject to forced evacuations, road closures, road openings and anything else we need to do as a state to keep our residents and to keep property safe. We don't take these decisions lightly," Ms Berejiklian told reporters on Thursday.
Those evacuating from Batlow are advised to travel north towards Wagga, through Adelong via Batlow Road and Old Tumbarumba Road, then the Hume Highway and Sturt Highway.
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