Paymark spokesman Paul Brislen said the outage occurred at 8pm when "a component within the Paymark network experienced connectivity issues".
Paymark restored service at about 11pm and transactions were now being processed as normal.
Shopper Donna Tawhara said the Eftpos failure was announced over the loudspeaker at Pak'nSave Glen Innes in Auckland.
"[They said] it was a nationwide problem and [we] must have cash to pay," she said. "They suggested we use the Westpac ATM machine in the foyer.
"There was a long queue to get the cash. Tried my card first but it said 'Card not allowed'."
Hamish Hung was at Victoria Park New World in central Auckland during the outage.
"Kids cried since they started to get hungry, a few old ladies yelled and complained, then blamed the supermarket for poor service. Some others at the end of the queue got extremely confused then tried to call any staff nearby to find out what was happening. It was complete chaos."
At Pak'nSave Manukau, Mark Tuifao said he had had $160 worth of groceries "processed, scanned and ready to go".
"The attendant swiped my card, only for it to be timed out. This happened another two times until the attendant realised other counters were experiencing the same issues.
"The supervisor comes to the counter and says, 'What bank are you with?' Not sure that was the answer, given that most of those paying had issues and were from different banks."
Sophie Aitken, a worker at New World in Thorndon, Wellington, said "all of a sudden all Eftpos machines went offline at once".
"This Eftpos crisis caused me, as a staff member, to stand outside alerting customers to the fact that only cash could be accepted. People's faces dropped at the word 'cash' as no one carries cash these days, so it was definitely the most action ATMs had seen in a while."
Paresh Jesani said he was at the Auckland Airport McDonald's. "I tried four times but still couldn't use it and then the service woman said that 'I will just give you what you wanted and don't worry about paying', therefore I got free stuff."
Richard Bol said he was having drinks with his partner Michelle at Grace O'Malley's Irish Inn in Whitianga and when they went to pay, their cards didn't work. The bar let them have the drinks for free.
In Highland Park, East Auckland, Kay Byrne said petrol-station staff were telling drivers, even before they got out of their cars, that they needed to have cash.
"Never seen Highland Park so busy," Ms Byrne said. "Some customers are waiting in the carpark ready for Eftpos to work again. Long night ahead for store workers."
At a KFC fast food restaurant in South Auckland customers were made to sign receipts before they were handed their order.
Deidah posted: "Wait, so Otahuhu KFC are making people sign their receipts and be charged tomorrow because their Eftpos machines are down."
