John Holland was awarded the design and construction contract for the Cronulla Line Duplication by the Transport Infrastructure Development Corporation. The Cronulla Line Duplication project was part of the NSW Government's Rail Clearways Program with the purpose of improving the capacity and reliability of the metropolitan rail network.

The Cronulla Line was previously a single track with a double track section between Caringbah and Gymea to allow City and Cronulla bound trains to pass each other. This limited the capacity for services on the Cronulla Line to four trains per hour in the peak periods.

The duplication of the Cronulla Line increased the frequency of trains to two additional services to operate per hour during peak periods. The separation of the City and Cronulla bound trains improved the reliability of the service by being less vulnerable to line disruptions.

• the duplication of 8km of existing line between Sutherland, Gymea, Caringbah and Cronulla
• the reconfiguration of the Sutherland Junction
• new stabling yards at Cronulla
• re-signalling, new communications and electrification of the entire line
• upgrades to Sutherland, Kirawee, Woolooware and Cronulla Stations
• eight substantial road and rail bridge modifications, and
• extensive track possession works, and environmental and public domain works.