Sydney's planned Western Harbour tollway will expose some north shore and inner-west residents to heavy construction noise, and produce a "plume" of toxic sediment in parts of Sydney Harbour, according to a preliminary environmental study seen by the ABC and Fairfax Media.

Key points: The project will include tunnels, tollways and bridges between Rozelle and Balgowlah

The project will include tunnels, tollways and bridges between Rozelle and Balgowlah It will cost about $14 billion and be completed in 2024

It will cost about $14 billion and be completed in 2024 White Bay will host a plant where concrete for the tunnels will be made and shipped across the harbour

White Bay will host a plant where concrete for the tunnels will be made and shipped across the harbour Residents worst affected by long-term noise may be relocated or "bought-out"

The high-level study marked "cabinet-in-confidence" was prepared for the NSW Government ahead of its announcement of the preferred route for the $14 billion project, set to be completed in 2024.

The consultants canvassed four alternative routes including a replacement for the historic Spit Bridge in Mosman.

They concluded the final route would deliver the most time savings to commuters and the least disruption to local residents, while highlighting environmental concerns including:

Noise impact on residents in Balmain, Pyrmont and Rozelle

Noise impact on residents in Balmain, Pyrmont and Rozelle The need to dredge and remove half-a-million tonnes of contaminated sediment, mostly in White Bay

The need to dredge and remove half-a-million tonnes of contaminated sediment, mostly in White Bay Resulting toxic "plumes" of water that could impact aquatic life

Resulting toxic "plumes" of water that could impact aquatic life Potential impacts on recreational harbour users, shipping and tourism.

A State Government spokesperson told the ABC the project would reduce traffic congestion across Sydney, and return "local streets to local communities".

The ABC understands a full Environmental Impact Statement (EIS) is being prepared for the Government.

The EIS, final project costings and various toll options are expected to be released for community consultation later this year.

Much of the tollway will run underground, with one tunnel between Rozelle and Waverton, and another running from Crows Nest to Balgowlah.

Two undersea crossings near Balmain and Seaforth will be created from water-tight concrete tubes laid on the harbour floor.

Prepared for NSW Roads and Maritime Services, the document reveals a casting plant will be built on wharves at White Bay to fabricate the large concrete sections used in the undersea tunnels.

A noise-modelling map shows the suburbs of Balmain, Pyrmont and Rozelle face "noticeable" impact from the casting plant.

Diagrams included in the study show the plant will occupy about 25,000 square metres and generate industrial noise and vibrations for up to 12 hours a day.

The site will also host a freight dock, where the components will be shipped on barges to work sites around the harbour.

Noise imaging shows the impact (green) of heavy construction noise at the work sites (orange) in and around White Bay. ( ABC News )

The White Bay site will also be a major hub for trucks carrying equipment and materials.

The document shows other sites for heavy trucks and work teams will be located at:

St Leonards Park, North Sydney

St Leonards Park, North Sydney ANZAC Park, Cammeray

ANZAC Park, Cammeray Flat Rock Baseball Diamond, Naremburn

Flat Rock Baseball Diamond, Naremburn Spit Reserve, Mosman

Spit Reserve, Mosman Yurulbin Park, Birchgrove

Yurulbin Park, Birchgrove Koobilya Street BMX Park, Seaforth.

The study warns that where serious long-term noise cannot be mitigated at construction sites, "property buy-outs" or relocation of "worst-affected residents" may be necessary.

The document does not predict those locations, but notes all compensation negotiations will accord with the Roads and Maritimes Land Acquisition Policy.

Disposal of toxic sludge poses health risks

The study outlines in detail the potential environmental risks posed by undersea construction.

The main concern is the removal of toxic sludge, a legacy of decades of industrial activities around the harbour.

The document confirms over half-a-million cubic metres of contaminated sediments must be excavated by dredging teams, most of it around White Bay.

The sediments include known carcinogens such as PCBs and dioxins, together with heavy metals.

In the Middle Harbour, the potential contaminants are of lesser concern, but include heavy metals, pesticides, and tributyltin, a chemical used in shipworks.

A number of disposal solutions are outlined in the document.

The options include dumping the sludge further offshore, disposing of it by land, or depositing in "deep holes" within Sydney Harbour itself.

Much of the material is expected to be classified as "controlled waste," which requires the NSW EPA to authorise any disposal plan.

Contaminated 'plume' could harm tourism

The document warns that dredging at Middle Harbour and near Birchgrove could also create a "plume" of turbid, contaminated water.

Thermal maps contained in the document illustrate the potential spread of the two plumes, near popular waterfront areas.

As weather, currents and tidal conditions are expected to influence the plumes, the document recommends detailed studies are carried out over a 4-kilometre radius at each location.

A thermal map charting spread and density of turbid plumes around Seaforth and Northbridge. Red indicates higher density. ( ABC News )

Thermal map: Sydney Harbour will be affected by plume, where the road crosses from North Sydney to Balmain. The density will be highest in the red region. ( ABC News )

The report also recommends the use of silt curtains — large, industrial screens of fabric — to help reduce the plume during dredging.

The curtains, suspended deep in the water, can be used to surround dredging equipment, or to protect sensitive areas near the site.

Recreational harbour users are also expected to be impacted by the plume.

Sailors and snorkellers are singled out for concern, with the document saying "increased turbidity may have adverse aesthetic and health and safety impacts on those users".

The document also says "tourist attractions" may be affected by the plume, temporarily threatening Sydney Harbour's status as an "iconic location".

Threats to aquatic life

The report states that more than 70 threatened species are listed as at risk from the project, including fragile seagrasses which support more than 20 species of endangered seahorses and sea dragons.

Rare fauna, such as the critically endangered black cod, dolphins, sea turtles and little penguins could also be affected.

"Connecting habitats" used by many species of juvenile fish may also be affected by dredging.

A spokesperson for the NSW Minister for Roads, Maritime and Freight, declined to answer the ABC's specific questions about the project, stating the Government's "homework with the community" is expected to be completed in the middle of 2018, allowing consultation to take place.

"We have always said we would come to the community then to discuss the next set of details," she said.