The work will be carried out on the Stanwell Park viaduct, which sits just south of that suburb's station.

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The hundred-year old viaduct was officially opened in 1920 and is now in need of some substantial maintenance work.

Over the next five years, Transport for NSW will spend between $13 million and $15 million to ensure the long-term safety of the rail bridge.

"Sydney Trains undertakes routine maintenance of the Stanwell Park viaduct and is planning to start major maintenance work, which includes renewing one of the spans and refurbishing the other seven spans, from the next financial year (2021-2022)," a Transport for NSW spokeswoman said.

Span six is the one that will be replaced during the maintenance work.

"The Stanwell Park viaduct is an important piece of infrastructure needed for current and future rail services between the Illawarra and Sydney."

The spokesman also noted the corridor between Sydney and Bomaderry has been identified as one of four potential routes identified as the starting point for investigation into a Fast Rail network in NSW.

The majority of the maintenance work on the viaduct will be carried out during planned weekend trackwork possessions and closedowns to limit the impact on commuters and freight.

But the money being spent won't result in a faster train trip through the area.

Wollongong MP Paul Scully quizzed Transport and Roads Minister Andrew Constance on whether the refurbishment will result in trains travelling across the viaduct faster than the current 60km/h.

"There are no plans to increase the speed at which trains travel over Stanwell Park viaduct after completion of these maintenance activities," Minister Constance said.

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