The Cronulla train line will be shut down in the July school holidays while one of the rail bridges at Gannons Road, Caringbah, is replaced.
Buses will replace trains between Sutherland and Cronulla from Saturday July 6 until Sunday July 14.
Gannons Road will be closed to traffic between Kingsway and Denman Avenue from 10pm on Friday July 5 until 4am on Monday July 15. A shuttle bus will be provided for pedestrians by Sydney Trains.
The work will widen the underpass, which is a major traffic bottleneck.
The state government is carrying out the bridge replacement, with Sutherland Shire Council to complete road works.
The entire project is due to be completed by the end of the year.
The original bridge was built in 1939, while a second bridge was built in 2010 when the Cronulla line duplication was completed.
At present, there is only only one lane southbound under the bridge.
The state government announced the widening in December 2017.