Greens NSW have obtained a May 2016 report commissioned by the Office of Heritage and Environment that recommended the government begin a scoping study in time for a world conservation congress later that year in Hawaii. It is not clear if any such study was done.

The prospect of a nomination for "Nasho" - the 16,000-hectare park to Sydney's south that in 1879 became is the world's second national park - had been touted by several Coalition environment ministers, starting with Robyn Parker.

"There is tremendous public support for World Heritage listing and the NSW government will be asking for Commonwealth assistance to prepare a nomination," Ms Parker said in a media release in September 2013.

Rob Stokes, Ms Parker's successor, said in a separate statement 18 months later that the government remained committed to researching a possible nomination.

"Royal National Park is one of the most remarkable places in Australia and we are currently finalising our assessment of the potential World Heritage values of the park," Mr Stokes said.

Gabrielle Upton, the current environment minister, declined to comment on the nomination's progress. "The NSW government is reviewing advice on the matter," her spokesman said, without elaborating.

Dr Faruqi said the government should explain why it had failed to pursue World Heritage status for the park. The next deadline for nomination is February 1 next year.

She highlighted recent Fairfax Media reports the government was mulling buying 60 hectares of the Royal National Park for $40 million to make way for the proposed F6 expressway linking Sydney to the Illawarra. The alternative is spending multiples of that amount to acquire and bulldoze hundreds of homes.

"With the recent revelations that this government is planning to put a motorway through the Royal National Park, one is compelled to question, is this why World Heritage status has not been pursued?" Dr Faruqi said. "What is the government hiding behind the smokescreen of undelivered promises and lies?"

The consultant's report, obtained by the Greens under freedom of information laws, outlines the complex process needed to win support for World Heritage status.

It "strongly argues that a scoping study is warranted", a step that would lay the groundwork for a separate thematic study to convince the World Heritage Committee.

The report outlines the merits of the park, including its status as being founded just seven years after Yellowstone National Park in the US.

It also noted the Royal National Park - initially known simply as National Park - was initially set up for recreation and the "acclimatisation" of exotic plants.

The Official Guide to the park, published in 1902, praised the area's natural wonders while ignoring the presence of Indigenous inhabitants.

"Some of the gorges are thickly studded with majestic trees, which for centuries have grown in solemn silences unbroken by man's footfall", according to the book, republished in 2013.