The alternative, according to an internal government report, is the acquisition and bulldozing of about 460 houses and 40 commercial properties between between Loftus and Waterfall at the cost of hundreds of millions of dollars.

The Roads and Maritime Authority believes it can acquire the 60 hectares of the Royal National Park for about $40 million.

Fairfax Media has seen the property acquisition and communications plans for F6 motorway, planning funds for which will be included in next week's budget.

Read more: $15 million funding boost for F6 extension

The documents indicate that construction of the motorway could start as soon as 2019, and they include a step-by-step guide to managing community relations as it is planned and built.

The say messaging about the national park will "need careful management".

The 16,000-hectare national park is Australia's first, and the second in the world after Yellowstone National Park in the United States. The Commonwealth Department of Environment says it is home to one of the richest concentrations of plant species in temperate Australia. It was placed on the National Heritage List in 2006.

The communications plan says Roads and Maritime Services has briefed the Office of Environment and Heritage on topics including the "Royal National Park" and "Sensitive Georges River Wetlands".

The government would only need to acquire about 60 private residential properties to complete the rest of the tollway between St Peters and Loftus at an estimated cost of about $120 million, or $2 million per house. Most are in Sans Souci and Miranda. Many of the addresses are in Meriel Street, Sans Souci, Taren Point Road, and Gwawley Parade Miranda.

About 200 properties at present zoned for roadworks would not be required and could have the zoning lifted.

The Herald applied for but was denied access to the document under the Government Information (Public Access) Act on the ground that it was prepared for the dominant purpose of being submitted to cabinet, "and was in fact submitted to Cabinet".

A spokesman for Roads Minister Melinda Pavey said on Tuesday that no decision had been made regarding the final alignment or form of the F6 Extension.

"The government has not yet decided when corridor recommendations will be made," he said. "The community will continue to be kept informed at key stages of the project."

Next week's budget will allocate a further $15 million for planning work on the F6. In December the government allocated $20 million for geotechnical testing.

The government has not confirmed when or if construction will start on the project. The documents seen by Fairfax, however, assume that an environmental impact statement will be prepared soon.

They say it will take a minimum of 18 months to acquire each property, either by negotiation or compulsory acquisition, should agreement not be reached.

"Given current public and political sentiment arising from the WestConnex property acquisition program, there is unlikely to be much political appetite for reducing the duration of the acquisition program," the acquisition plan says.

A Transport for NSW memo released earlier under the Public Access Act says the difference between the cost of the proposed F6 Extension and an alternative cheaper proposal that would cut the time taken to travel from Sydney to Wollongong by train from 90 minutes to 65 is "sufficient to construct the equivalent of almost three Snowy Mountains Hydro Schemes".

The communications plan says if the alternative of an improved rail connection is raised by the media or stakeholders, the appropriate response is to say that although the idea was commended in an earlier transport master plan, "subsequent studies have not been supportive of an immediate start".

A draft Q&A for fielding questions from the media avoids mention of the rail proposal of the Royal National Park saying "the study is in its early stages and the community will be kept up to date as plans progress".

It says it is "too early to speculate" on the potential cost or on whether the road would be tolled.

The six-phase communications plan would begin with a "low key, matter of fact" letter to residents affected by geological testing that does not mention the F6 extension.

"Key influencers / key commentators" would be given early one-on-one briefings as soon as practicable.

The briefings would be confidential, "but would allow the senior stakeholders to provide informed comment on the project if the need arises", the communications plan says.

Among the named key influencers are Paul Forward, a former head of the Roads and Traffic Authority identified as an "influential transport commentator", the head of Infrastructure Partnerships Australia Brendan Lyon, identified as a "thought leader", and the head of the Sydney Business Chamber Patricia Forsythe, identified as a "strong supporter".

Asked on Tuesday whether the F6 Extension would be built, Premier Gladys Berejiklian said her government loved "the challenge of building projects that have been in the too-hard basket for too long".

smh.com.au