A damaged rail line from Whitehaven Coal's Narrabri mine in north-west NSW will re-open five days earlier than expected.

Trains have been unable to move coal between the Narrabri underground mine and the Port of Newcastle since the track was damaged by a derailment on 28 November.

The Australian Rail Track Corporation had advised the line would reopen on Christmas Day, but that has been revised to 20 December, Whitehaven said on Friday.

The company has applied for approval to truck coal from the Narrabri mine to a plant at Gunnedah, which is still being assessed, Whitehaven said.

A small scale trial of trucking has highlighted issues with the amount of freight using the route, and the company is working to address those problems in the event approval is granted for full scale trucking.

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