And, according to Transport for NSW, it is unclear exactly when commuters will see that extra service.

Last month Transport Minister Andrew Constance visited Helensburgh station to announce "thousands" of extra seats on the South Coast line.

This would be achieved by turning eight-car trains into 10 cars.

There would also be an express train every 15 minutes between Wollongong and Sydney during the morning and afternoon peak.

In real terms that means one extra morning train arriving at Central between 8am and 9am and one in the evening, leaving Sydney between 5-6pm.

A Transport for NSW spokesman could not give a specific time for when the extra two peak-hour services will appear.

"Planning for future stages of the More Trains, More Services program is currently under way, with service improvements expected to commence from the early 2020s," the Transport for NSW spokesman said.

"The program includes service improvements for the T4 Illawarra and South Coast lines, and we will keep our customers informed well ahead of any changes."

The spokesman said planning was also under way for extra services during the broader peak periods but did not give a time frame for when they would be rolled out. 

"Across the entire week, the combination of additional carriages and services on the South Coast line will create an extra 13,000 seats for customers in the AM and PM peak," the Transport for NSW spokesman said.