Victorian government reveals their tunnel vision for new CBD rail project

The Victorian government has revealed the Melbourne Metro rail tunnel will run underneath the Yarra River, 11 metres below the waterline.

Engineers on the multi-billion dollar project will use two tunnel boring machines which will be buried underground.

The animation makes it look easy, but Transport Minister Jacinta Allen concedes tunnelling under the Yarra will be a difficult job.

"Crossing the Yarra is one of the more complex parts of the Melbourne Metro rail project," Ms Allen said.

Tunnel Boring Machines (TBMs), use a specialised cutting head to grind through the ground, then immediately seal the excavated tunnel with concrete.

Using the TBMs to cross the Yarra differs from what was proposed in the previous Metro Rail business case, which suggested an immersed tube tunnel or coffer dam construction.

"Both of those approaches were seen as being more disruptive to the environment, more disruption to surrounding businesses," Ms Allen said.

"It's not a question of cost or a difference in how much more expensive one option is."

Pressure is on to get it right, with previous tunnelling efforts under the Yarra proving problematic.

Construction of the Burnley Tunnel in 1999 resulted in serious leakage issues.

Public consultation is now underway.