Two-hour myki fares will be crunched on Sunday to exactly two hours' duration from when a passenger touches on, giving Victorians less travel time before they must pay more for an all-day fare.

Currently, two-hour fares expire at the end of the hour, meaning a ticket can last as long as two hours and 59 minutes.

Public Transport Victoria posted a note on its website on Friday morning but has made no other public announcement, sparking accusations that it has not given the public proper warning of the important change.

Sunday also marks the start of on-the-spot fines, when fare evaders can pay $75 immediately or be slugged $217.

Public Transport Users Association president Tony Morton said the authority was being "sneaky".