Buses are replacing trains on the Sunbury line between Sunshine and Watergardens due to a train fault near St Albans.
Train services are operating between Flinders Street and Sunshine, and between Watergardens and Sunbury.
The Department of Transport is telling commuters to allow up to an extra 30 minutes for their journeys.
The 420 bus route is being recommended as an alternative transport mode.
Details: www.ptv.vic.gov.au or in the PTV mobile app.