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.
"We feel that as a matter of courtesy to Victorians there should have been at least a week's notice given," Dr Morton said.Public Transport Users Association president Tony Morton said the authority was being "sneaky".
"They have found it's ready to roll out so they want to do it as soon as possible, but really for the sake of giving a week's notice rather than two days' notice the extra revenue they would get from that is negligible."
The Napthine government announced the impending fare change in December, but its introduction, which was originally scheduled for July, was delayed because the myki ticketing system was not prepared.
Dr Morton predicted many people would be caught out from Sunday, and said this could have easily been avoided had the authority spread the word properly.
"There should have been more of an awareness campaign," he said.
"It's just a little bit sneaky and it doesn't give public transport users a good feeling about how the system is supposed to operate on their behalf when these kind of changes are rolled out under the radar."
A Public Transport Victoria spokesman said the authority had alerted people on July 1 that the fare change would happen in August.
"On July 1 this year, PTV published on its website that two-hour myki fares would commence in August," he said.
"PTV published information on its website today to remind customers of this change which comes into effect on Sunday."
A full fare two-hour zone one ticket costs $3.58; a zone one-and-two ticket costs $6.06. A full fare all-day ticket costs $7.16 in zone one; $12.12 in zones one and two.
Dr Morton said the Public Transport Users Association opposed the change because it would make the system less user-friendly.
"We think it's a bad move mainly because it means that people travelling in the daytime on a two-hour fare will have virtually no idea of when their fare is going to expire," he said.
Read more: [color=#003399][font=Arial, Helvetica, sans-serif]http://www.theage.com.au/victoria/sneaky-twohour-myki-change-buys-you-less-travel-time-from-sunday-20140808-101uzh.html#ixzz39nlCc1bU[/font][/color]
Great notice given by PTV by fare changes!