The discovery of the utilities snaking underground was one of the main reasons the project was delayed under the previous Labor government.

The cables and pipes raised the project's expected costs and lengthened the construction timing.

The utilities lengthened the project so much that it had still not begun when the government changed in 2011 and the Liberals' revamp of accessibility funding saw Unanderra moved back down the queue.

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A Transport for NSW spokeswoman said water, optic fibre, gas and electricity services were located under the proposed lift shafts.

Some of these will need to be removed while others can remain in place, albeit with some remediation work.

"Subject to design, community consultation, and planning approvals, utility services will require relocation or protection," the spokeswoman said.

"The current concept design, has low voltage electricity and signal cables that would need relocation.

"Nearby water, high pressure gas main, communication and signalling cables will require protection during the works."

Transport and Roads Minister Andrew Constance has promised $20 million for the Unanderra station upgrade, which would be delivered in this term of government.

The Transport for NSW spokeswoman said it was unclear how much the utility relocation will add to the cost of the project.

"The cost of service relocations will be determined once detailed design has been completed and a contractor appointed to deliver the project," the spokeswoman said.

The Unanderra upgrade will feature three lifts - two on either side of the tracks and a third to deliver passengers to the island platform.

Improved pathways and a family accessible toilet are also likely to feature in the station upgrade.