That's not because the government has gone back on its promise - which was to build it in this term of government - but because construction is not yet ready to start.

According to Transport for NSW there is still more work to be done on the plans for the three-lift upgrade at Unanderra before shovels can hit the dirt.

"Further technical investigations and planning work, including service investigations, design, geotechnical surveys and constructability assessments, are continuing at Unanderra station," a Transport for NSW spokeswoman said.

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"Once a final concept design has been completed the formal planning document, the Review of Environmental Factors, will be developed along with the required specialist studies."

Generally, a project will only get budget funding once these steps are all completed and the construction is ready to begin.

Otherwise the $20 million will sit there idle while other projects that are ready to go don't receive any funding.

Wollongong MP Paul Scully has previously demanded the funding for Unanderra be in Tuesday's budget, but that seems unlikely to happen.

It is unknown at this stage whether there will be funding in the budget for the two-lift upgrade at Fairy Meadow station.

At the moment it would appear to be ahead of Unanderra in the queue as the review of environmental factors has already been on public exhibition.

This means a decision on construction dates for Fairy Meadow could be made some time this year.