That means those travelling for fewer than four trip stages are penalised, but those staying aboard longer stand to gain - until their cards are blocked.
Auckland Transport says it relied on default charges for 861,340 transactions made on Hop cards to the end of October, from a total of 92,844,300 transactions.
Total card forfeitures to that date meant the confiscation of $34,724.16c in credit, of which the council body says only a "minimal" amount has been transferred to replacement cards.
It says it cannot provide a figure for that without manually researching each case.
Cards also have to be registered before it will investigate whether they were blocked through no fault of their users, as in a tiny number of reported cases of machine failure.
Auckland Transport says 624 of the cards it has blocked - carrying credit of $6303.33c - were unregistered.
It meanwhile refuses to disclose under official information legislation the number of missed tags it allows passengers to run up before it invalidates their cards.
AT Hop group manager Eunan Cleary said in reply to an information request from the Herald it was keeping that detail confidential so passengers could not use it "for improper gain or advantage".
But a ferry passenger has told the newspaper her card was invalidated after three failures to tag off over several months last year.
She said that was despite an acknowledgment by officials that a machine where she had been unable to tag off at the Birkenhead ferry terminal had been faulty on those occasions.
The woman said she had elected for privacy reasons not to register her card, so was told she could not be reimbursed for about $8 of unused credit left on it.
Mr Cleary said registered Hop users receive email notifications about missed tags before their cards are blocked.
"Auckland Transport monitors all card usage to ensure customers are using their cards in the prescribed manner," he said.
"Where anomalies are noted, we contact registered card-holders to advise of the error and explain the correct process to ensure their card is not subsequently blocked."
Another Auckland Transport spokesman was puzzled by the passenger's recollection that her first and third missed tags were months, rather than weeks, apart.
The organisation's policy was to disable cards only for a certain number of failures within "a short time" of one another, which he would not define. He said he could not check the veracity of the woman's claim, given that her card was unregistered.
Hop numbers
• Number of Hop card transactions from November 2012 to October 2014 - 92,844,300
• Number of transactions where passengers have missed tagging their cards - 861,340 (0.93%)
• Total revenue from Hop card transactions - $104,590,584
• Number of cards invalidated by repeated tag failures - 3239
• Stored credit locked on invalidated cards - $34,724.16c
