Seniors groups say the decision will also be of "huge concern" to pensioners who do not want their Opal cards linked to their bank accounts.

"This goes against everything that a good public transport system should do," said Charmaine Crowe, a spokeswoman for the Combined Pensioners and Superannuants Association. "That is to make transport easy and not a hassle."

On Monday, Transport Minister Gladys Berejiklian announced that temporary pop-up kiosks would start selling Opal cards at large stations and on selected days until the end of September.

But her department also confirmed for the first time that there were no plans to allow the cards, which are being introduced at a cost of $1.2 billion, to be permanently sold or topped up at Sydney Trains' existing ticket windows or stations.

This disclosure shocked many, with the opposition accusing Ms Berejiklian of wasting money by hiring temporary sales staff instead of allowing existing employees to sell Opal cards.

People can currently buy or top-up Opal cards online or by phone. The cards will eventually be sold at retail outlets near train and bus stops which currently allow people to top-up their cards. But some stations do not have convenient retail outlets.

Anyone wanting to top-up their Opal card at Banksia station, for instance, would need to walk half an hour to Kogarah station to top up their card, according to Transport for NSW's map of retailers.

The nearest retailer to North Ryde station is a newsagency at Macquarie Park, about 25 minutes' walk away.

The closest retailer to Lewisham station is a 14-minute walk away. The busy Wolli Creek station does not have a retailer within less than a 10-minute walk.

"Unless there's a booth right there that can help you, you are going to have to find somewhere else … that could be some distance away or it could be closed on Saturdays," said Jim Donovan, of Action for Public Transport. "You might need to cross the street in the rain and that's inconvenient."

From September, the government will stop selling 14 types of public transport paper tickets at rail stations, including the adult off-peak return ticket.

This means that an adult without an Opal card wanting to travel outside peak hour will no longer receive a discount.

The government is trying to encourage people to link their Opal cards to their credit cards, which allows them to be topped up automatically. The majority of existing Opal users take this option.

But Ms Crowe said pensioners were concerned about having money automatically withdrawn from their accounts.

"We've been trying to assure them that they can do manual top-ups," she said on Monday night. "But this obviously makes manual top-ups far more difficult, which I think is going to be of huge concern to a lot of pensioners."

The head of the NSW Rail, Tram and Bus Union, Alex Claassens, said he could not understand why Ms Berejiklian would not get existing staff to sell the Opal card when Sydney Trains was making sales positions redundant.

"I just made an assumption that that's what would happen," Mr Claassens said. "Long term, why wouldn't you have them available to be sold at a railway station?

"It doesn’t make sense."

Fairfax Media asked Ms Berejiklian and Transport for NSW for a straight answer as to whether they would set up vending machines allowing top-ups at stations.

The department responded that it would "continue to look at ways to make Opal cards available/accessible for customers as the roll out progresses".

The government says cards bought from kiosks will enable anonymous travel.