The cost to taxpayers of building the first three stages of the proposed F6 Extension between Arncliffe in Sydney's south and Loftus has been put at more than $9 billion.

The figures, obtained from leaked cabinet-in-confidence documents, also estimate the government will be on the hook for an extra $2.2 billion in operational costs for the tollroad over 35 years.

Under the plans mirroring those for WestConnex, tolls for each of the three sections will range from about $3.50 to $3.80 – capped at just over $8 for a one-way trip along the entire motorway – in today's dollars and rising annually at the rate of inflation or 4 per cent, whichever is greater.

The leaked documents, obtained by Fairfax Media and the ABC, help explain why the government last month shelved plans to build a final stage between Loftus and Waterfall.