The preliminary investigations will take place inside the rail corridor near Bomaderry Station and Toolijooa and are expected to take five weeks to complete.

The information gathered will help develop the design the proposed rail loop and involves drilling boreholes, digging test pits and underground service investigation.

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Kiama MP Gareth Ward said the line was one of four routes being investigated to link regional centres to each other and Sydney through fast rail.

"The work is the first critical step in delivering a crossing loop near Toolijooa, which will duplicate the rail line between Berry and Gerringong and transform journeys along the South Coast," he said.

"In the future this will mean more frequent services for South Coast customers, as trains will be able to pass each other between Kiama and Bomaderry.

"These upgrades will support the capacity for a service every 30 minutes in peak hours and every 60 minutes in the off-peak, potentially doubling the frequency and providing more connections to Wollongong and Sydney.

"An extra platform is also planned at Bomaderry Station that will allow a train to arrive before another departs."

Work was carried out on Saturday, August 1 and Sunday, August 2 between 7am and 6pm, and will continue on weekdays from 7am to 6pm.