More than a year of road chaos looms for a busy CBD street so a century-old sewerage system can be duplicated to cope with extra human waste brought on by a surge in apartment buildings.

The increase in apartment buildings at the CBD's western end will force the partial closure of Spencer Street from next week.

Vicroads says hiring traffic management staff cost it more than $700,000. Credit:Jessica Shapiro

The works will last for up to 15 months and will see the shutdown of half of Spencer Street's southbound lanes, from Little Lonsdale Street to the Yarra River, while City West Water lays almost 700 metres of extra sewerage pipes.

Spencer Street is one of the city's busiest pedestrian and car thoroughfares.