Rail commuters will face disruptions on the South Coast line until next Friday with buses replacing trains until next Friday (18 July).

Buses will be used instead of train services between Thirroul and Sutherland.

Express buses will also run between Wollongong, North Wollongong and Sutherland, where passengers travelling to the City can join a T4 Illawarra rail service.

T4 services will continue to run between Bondi Junction and Cronulla and Waterfall.

Sydney Trains is carrying out planned critical upgrades and essential maintenance work.

The South Coast Line is vulnerable to bad weather due to its topography, particularly between Waterfall and Thirroul.

In a statement, Sydney Trains said "we are proactively working to improve the resilience of the South Coast Line with our ongoing maintenance programs".

"Sydney Trains undertakes an extensive maintenance program across the rail network to ensure passenger journeys continue to run smoothly and safely.

"To minimise disruption to passengers, the work is being carried out during the school holidays when fewer people are travelling on the network.

"Sydney Trains apologises for the inconvenience and thanks passengers for their patience as we carry out this essential work."

The work includes ballast cleaning between Waterfall and Coalcliff, cutting and embankment works at Waterfall and Scarborough, cyclic track maintenance at Helensburgh, drainage and vegetation maintenance, and defect removal and routine inspection for electrical, signal and track assets.