The machines – each weighing 118 tonnes – have connected caverns that will become the new route's State Library Station. The overall project includes twin 9km rail tunnels and five new underground stations.
CYP Design & Construction - a consortium led by Lendlease, John Holland and Bouygues Construction - is carrying out the tunnels and stations package of works for the project.
The roadheaders have been mining station caverns and underground passenger connections under Swanston Street for the new station.
In total, seven roadheaders will be used in the central business district (CBD) as part of the project – four for State Library Station and three for Town Hall Station. Each is 15 metres long and is lowered underground in separate pieces, before being reassembled underground.
The three roadheaders have been working day and night for eight months and have now dug out more than 20% of State Library Station, with excavation expected to be finished by late 2020.
Transport infrastructure minister Jacinta Allan said: “This breakthrough is a huge achievement and the culmination of months and months of hard work. These giant roadheaders have been working day and night underground, building the Metro Tunnel – to run more trains, more often, across Melbourne.”