Documents seen by the ABC and Fairfax reveal taxpayers will be slugged almost $3 billion to complete a planned revamp of Central Station in Sydney.

Neighbouring commercial properties around Henry Deane Plaza and the Goods Line will be acquired as part of the ambitious 20-year building scheme by the NSW Government, including a heritage-listed post office building and a YHA hostel.

Central's Big Spend Upgrades to concourses and walkways: $1.4 billion

Upgrades to concourses and walkways: $1.4 billion Construction of underground Metro Station: $750 million

Construction of underground Metro Station: $750 million Commercial and retail developments around terminus: $650 million

Commercial and retail developments around terminus: $650 million High-rise development costs surrounding airspace: $350 million

Some construction is already underway and will span three decades, with major disruptions to passengers at Australia's busiest train station expected until 2035.

Upgrades at the main terminus, which dates to 1906, will cost $2 billion and include new platforms, shops, restaurants and a 180-room luxury hotel on the top floor.

The hotel plan requires the relocation of an existing light-rail stop to Pitt Street, creating a turning bay for limousines and taxis outside a new hotel foyer.

The secret Berejiklian government blueprint also warns commercial proposals may become unviable investments for the Government — recommending ongoing reviews as the project evolves.

New shops, restaurants and even a boutique hotel form part of the blueprint for Central Station. ( Supplied )

Skyline to be transformed

Over $1 billion is allocated to "commercial" and "aspirational" development projects near the station, transforming a low-rise skyline largely unchanged in over 100 years.

Artists' impressions reveal towering high-rises, some in the airspace over existing railway sidings.

Three commercial towers, at a cost of $300 million, will be built on Lee Street while railway sidings near Prince Alfred Park are earmarked for another $300 million project to build shops and low-rise residential apartments.

Developing the space will require the relocation of a large electrical substation at a cost of almost $40 million.

An artist's impression of Sydney's upgraded Central Station. ( Supplied )

Henry Deane plaza, a nearby shopping precinct, is also in line for a $100 million overhaul.

The documents show a 1903 heritage-listed post office building, a YHA hostel and other nearby properties would be forcibly acquired by planners, evicting tenants and repurposing the site for three office towers up to 40 storeys high.

A Transport for NSW spokesperson declined to answer questions on the documents, but confirmed that work on a new underground concourse and Metro station was well underway.

The spokesperson said that the NSW Government had made "no decision regarding further development of Central Station precinct" and future projects were "subject to rigorous economic appraisal and Cabinet consideration".

The documents warn much of the high-rise development is not yet "value for money", and recommends careful analysis of property markets over the next decade. The delivery of "economic profit" to developers is listed as a KPI of the project.

Overcrowding fears

The documents warn of dangerous overcrowding in pedestrian tunnels, as peak hour visitors to Central will rise by 70 per cent over the next two decades.

Central's busy platforms, and congested walkways, such as the Devonshire Street Tunnel, are expected to soon be unable to cope with pedestrian flows.

The documents recommend that Transport for NSW move urgently on upgrades to improve passenger amenity around Central, or risk delays to the opening of the Metro Project in 2024.

Transport Minister Andrew Constance has rejected the $3 billion cost estimate, stating that at present just $1 billion in taxpayer funding has been allocated.

He says an uncertain amount of extra funding will flow from property developers.

An artist's impression of Central Station's interior after its upgrade. ( Supplied )

"The bureaucrats are wrong, we will go to market and we'll find out what the numbers are" he said.

"Once we get the numbers — as to what the investment will be from the private sector — then we will know."

But Opposition transport spokesperson Jodi McKay says Mr Constance should reveal more about future plans.

"I think the minister should be honest about what is happening." she said.

"If his call is the bureaucrats have got it wrong, give us the plan and tell us what is costed."