Motorists trying to leave the CBD via St Kilda Road will endure even more delays, with a left turning lane into the road from Flinders Street to be closed for four years so that a new underground rail station entrance for the Metro Tunnel can be built.

An "imposing" acoustic shed set to be 18 metres tall will tower over Federation Square from next year, to allow for 24/7 excavation and tunnelling for the new Town Hall station.

Public Transport Minister Jacinta Allan has warned of 15-minute delays on St Kilda Road and some disruption to the tram network, as the government marches ahead with building a station entrance at Federation Square, despite the area recently receiving temporary heritage protection.

“These works will be disruptive but they are absolutely necessary to build the Metro Tunnel as safely and quickly as possible,” Ms Allan said.