The original rail bridge over Gannons Road, Caringbah will be removed in one piece during the July school holiday.
Sydney Trains said a 750 tonne crane would carry out the task, which is necessary for the road to be widened.
"The new bridge will be installed in pre-cast concrete sections with an in-situ concrete topping slab poured on site," a spokesman said.
The state government is replacing the bridge and Sutherland Shire Council will do follow-up road works, with the project due to be finished 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.
Calls for the original bridge to be replaced at the time of the duplication were unheeded by the Labor government of the day.
It has meant there is only one southbound lane under the bridge.
The state government announced the bridge widening in December 2017.
Last week's state budget provided $9.3 million for the project.
The budget also provided $109,000 to upgrade the footpath along Gannons Road, between Captain Cook Drive and Denman Avenue, to a shared cycle-walk path.
Sydney Trains is urging residents to familiarise themselves with temporary travel arrangements that will be in place during the project.
The Cronulla train line will be shut down, with buses replacing 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 for affected pedestrians will be provided by Sydney Trains.
"This vital project will greatly improve traffic flow, but it does involve some temporary disruption," the Sydney Trains spokesman said.
"Every effort has been made to ensure minimal disruption."