NSW Transport Minister Andrew Constance said buses would replace trains on the T1 North Shore & Western Line, between North Sydney and Wynyard, from January 1 to 10.

Commuters would need to plan their trips in advance as the work would affect almost every rail customer, he said on Tuesday.

"This work will ensure we continue to provide an efficient network for commuters and is a once-in-a-generation opportunity to extend the life of the 88-year-old railway corridor by 120 years," he said.

Sydney Trains acting chief executive Suzanne Holden said the maintenance involved replacing a timber deck with a longer-lasting concrete structure.

"This is a unique opportunity to improve the reliability of the rail corridor on a much-loved Australian icon," she said.

Transport for NSW chief operations officer Howard Collins said trains would run to a weekend timetable with additional peak services.

"While we traditionally see 40 per cent fewer customers travelling at this time of year, buses aren't able to match the speed or capacity of the train network, so we strongly encourage everyone to plan ahead, allow extra travel time and consider using the many alternative services we're operating," Mr Collins said.

Rail replacement bus routes will target the most popular trips and additional temporary trips will be added to the bus and ferry network.

Australian Associated Press