Sections of two rail lines in Sydney's south west will be shut over the Christmas holidays to allow for construction of a metro rail line, forcing up to 100,000 commuters a day to catch replacement buses.

Transport authorities plan to put on more than 200 buses a day to ferry about 100,000 people during the first four days of the closure of the Bankstown line between Campsie and Sydenham, and the Illawarra line from Hurstville to Central Station, from December 24 to 28.

Up to 90 buses a day will then operate for the rest of the closure of the Bankstown line until January 5, transporting as many as 28,000 passengers each day.

Commuters will be forced to catch buses during closures of the Bankstown line. Credit:Peter Rae

The project for the second stage of the multibillion-dollar metro rail line involves converting a 13.5-kilometre stretch of the Bankstown line.