With more than 2 million cubic metres of material set to be excavated during construction – equivalent to the capacity of 800 Olympic-sized swimming pools – the government has been advised that the project will need to be managed carefully to ensure that any possible danger is minimised.

"The potential interception of contaminated groundwater in the immediate vicinity of the project boundary during construction could lead to exposure risks to workers and the environment," the environmental effects statement for the project reveals.

"Additionally, the potential movement of contaminated groundwater due to construction/operation activities may impact on the beneficial uses of third-party property owners."

The Melbourne Metro Rail Project is a $10.9 billion cross-city tunnel between South Yarra and South Kensington, with five new underground stations. According to the environmental effects statement, possible dangers include asbestos in the soil along the project's route, vapours and gases diffusing into station boxes or tunnels, and exposure to contaminated groundwater by builders or residents.

The environmental effects statement also shows:

• Disturbance of ground and groundwater during construction may cause such gases as methane and hydrogen sulphide from Coode Island silt to "migrate" towards the tunnel.
• The management and safe disposal of potentially large volumes of rock and soil will be a "significant logistical issue" for the project.
• A "high probability" of acid sulphate soils emerging within the vicinity of the western portal site, Arden Street and between CBD South station and Domain station, which "may pose a risk to human health, buildings and structures, and the environment" if managed inappropriately.

Acting Public Transport Minister Luke Donnellan said Melbourne needed the rail project to run more trains in and out of the city, and the government made no apologies for getting it done.

"Excavating and managing soil is one of a number of challenges the Metro Tunnel presents, but we won't use them as an excuse not to build this critical project like the former Liberal government did," he said.

A spokesman for the Melbourne Metro Rail Authority added the agency would work with contractors once they were appointed to work out the most appropriate method of stockpiling material and, if required, disposal.

However, opposition spokeswoman Margaret Fitzherbert said: "It's either dishonest or poor planning to just keep saying that details will be worked out after the building contract is awarded.

"In June, the Andrews government announced the spoil stockpile will be in Domain instead of Fawkner Park. It should now explain exactly where it will be.

"If that kind of detail really hasn't been decided, the Andrews government needs to go back to the drawing board and plan this project properly."