The work, being carried out by Isaac Construction, is expected to be completed in April.
The roadworks involve old pipes being replaced, some of which date back to the late 1800s.
Road closures in London and Frederick Sts are part of the preliminary stage of a wider facelift for the CBD.
The upgrade plans are centred on upgrading George St, which is set to become a one-way street, following a council vote in September.
The overall upgrades are set to be completed in 2024.