This raises the potential they may replace existing services, as the state's pricing regulator investigates the way the state government approaches its on demand trials.

At the moment, there are two separate on demand bus trials operating in the Illawarra - in Thirroul and Shellharbour.

A third running in Figtree was scrapped in January.

The coverage areas for the Shellharbour trial was expanded earlier this year after the short-lived airport bus was cancelled.

Since their introduction in January last year, 26,872 passenger trips have been taken across the three services.

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The Independent Pricing and Regulatory Tribunal has warned government that services like these are not to be used as a substitute for other buses.

"On demand services need to be carefully designed to ensure that high-cost, low-patronage fixed route transport services are not simply replaced by even higher cost on demand services," the body said.

The trial in the Illawarra's northern suburbs covers an area that is already serviced by two bus routes - run by Dion's and Premier Illawarra.

A Transport for NSW spokeswoman declined to comment on the possibility that these existing bus routes may be replaced.

However, there was the suggestion it may make sense financially.

"Early indications are that on demand will be more cost-effective in the long term compared to how some services are delivered today," the Transport for NSW spokeswoman said.

"The objective of the On Demand program has always been to test and learn through the delivery of temporary pilots of new and creative transport services.

"We need to plan for the transport network of the future and that means providing upfront investment into new, cost-effective ways to deliver transport services when and where people need it."

The spokeswoman said Transport for NSW were no plans to introduce any extra on demand services into the Illawarra.