More than 1.5 million train commuters will be forced to find other transport when nine rail lines close for weeks in what Premier Daniel Andrews admits is an "unprecedented" disruption.

In the massive shut down, rail lines will be closed for more than three weeks, from April 1 to 23, with buses replacing trains on the Cranbourne, Pakenham, Frankston, Sandringham, Mernda, Gippsland and Geelong/ Warrnambool lines.

All rail lines passing through Richmond station will be shut over the three weeks, except for the Alamein, Belgrave and Lilydale lines. The Glen Waverley line will also be shut on some occasions.

Frankston, Cranbourne and Pakenham commuters will face the longest disruption, from April 6-23. The Frankston line will also be shut from April 1-3.