Starting this week buses will replace train services between Parliament and Camberwell stations, and Darling and Richmond stations on the Belgrave, Alamein, Lilydale and Glen Waverly lines.
The bus replacements are in place to allow for scheduled upgrades to improve signalling and power between Camberwell and Richmond stations. Some of the signalling is from the 1920s.
Peak-hour delays are possible from 8.30pm Thursday November 30 until the last scheduled service on Tuesday December 5.
Metro has advised travellers to allow an extra 70 minutes for their journey.
The upgrades come before a series of major upgrades over summer. These include work on the metro tunnel and on level-crossing removals.
Replacements will also be seen on Craigieburn, Pakenham, Upfield, Hurstbridge and Dandenong lines across summer due to level-crossing removal and station upgrades, so best to plan ahead when travelling to the CBD.