A major dredging operation to construct tunnels under Sydney Harbour and Middle Harbour holds major environmental risks, according to New South Wales Government documents leaked to the ABC.

Key points: The documents include a plan to manage community backlash with tailored messages

The documents include a plan to manage community backlash with tailored messages There are 76 threatened animal species near the two tunnelling sites

There are 76 threatened animal species near the two tunnelling sites The project will link Rozelle with the northern beaches

The plan outlines excavation, haulage and dumping of almost 130 million cubic metres of spoilage, as well as the risks of dispersing contaminants like heavy metals, PCBs (polychlorinated biphenyl) and dioxins through waterways.

Harmful pollutants are a legacy of decades of heavy industries using the harbour.

Risks are elevated in former industrial sites like White Bay, where 500,000 cubic metres of toxic sediment will be removed.

The document notes community concern over health impacts will be an issue.

Also recommended is the disposal of the spoilage at sea, as large volumes of sediments makes land-based option uneconomic.

Foreshores to become worksites

The closure to the public of foreshore areas is flagged at White Bay, Berrys Bay, Balls Head and around Middle Harbour for the construction of large worksites, including heavy equipment sheds on land.

Round-the-clock work and blasting of rock is expected during some phases of construction, and the document notes neighbouring properties may experience high levels of machine noise from both tunnelling and dredging silt.

The permanent destruction of maritime heritage could also occur.

The plan considers the removal of the historic Balls Head Coal Loader Wharf to allow barge operations and a work-plant to operate from the foreshore.

The historic Balls Head Coal Loader Wharf dates back to 1917. ( Flickr: Ian Sanderson )

The report advises that community opposition to dredging is a key issue, that could hold up or delay construction activities.

A spokesperson for the Roads Minister confirmed geotechnical work for the project was underway.

They said the final plan, expected next year "would be subject to a full EIS and would require the approval of the Planning Minister".

Plan to manage community backlash

Winning community approval for the entire project is a challenge recognised in the leaked Cabinet documents.

A separate Stakeholder Engagement Plan outlines a public relations strategy to government.

The plan lists residents and business owners, and users of open space, among "a hierarchy of stakeholders" with "ability to influence the project".

The two tunnels will be built under Sydney Harbour and Middle Harbour, linking Rozelle to the northern beaches. ( ABC News: Nour Haydar )

It also lists major "risks" posed by the stakeholders such as community and local government opposition, concern about falling property prices, and construction fatigue.

For each risk tailored messages are suggested. The messages include a "fresh approach" to property acquisition and an "awareness campaign" to show a "bounce" in property values from the finished project.

Social media, one-on-one meetings with individuals and "meet the experts" community sessions are also recommended to "manage emerging issues".

A public consultation meeting will discuss community concerns tonight in North Sydney.