At multiple Sydney stations over the past fortnight, large crowds have developed after Opal card readers at the gates malfunctioned.

In some instances, they have compelled Sydney Trains staff to throw open the gates for anyone with a paper ticket.

Transport for NSW continues to describe the problems as “minor”, and a large number of Opal card users seem to be having little trouble.

But issues are regularly emerging at busy stations at peak periods.

They appear to relate to the display panels that show commuters how much credit they have remaining on their Opal cards.

Fairfax Media has seen evidence from multiple train stations this week that many ticket readers are not working properly on a regular basis.

If commuters do not see their balance shown correctly on the reader at the gates after tapping their Opal cards, they may not want to move through the gates and risk attracting a maximum $8.10 fare.

It may mean that they have been overcharged or that they are unsure what they have been charged.

The confusion causes delays as other commuters bank up behind.

In a statement provided late on Friday, Transport for NSW said there was no technical issue with the Opal system overcharging customers or charging them a maximum or “default” fare.

According to the department, people might think they are being overcharged, but the Opal system has just not yet applied the proper fare.

“The only reason customers will get a default fare is because they have not tapped on or off,” the statement says.

The Opal system works by holding a default fare – in the case of an adult train user $8.10 – against someone’s account until it updates with the proper fare.

“Whenever a customer travels, the system holds a default fare from their value and this is what some customers may see if they view their Activity Statement soon after travelling,” the statement says.

“Opal journeys are usually recorded in a customer’s Activity Statement within 24 hours and in rare cases this may take longer. The system will adjust the statement for the proper fare over this time.”

Sydney Trains staff have display panels that show them which gates are working well. At one busy central business district station on Thursday, the panels showed most gates had problems.

Images provided by the Rail, Tram and Bus Union demonstrate this is a regular occurrence.

One gate staff attendant said the problems had been going on for over two weeks, and they regularly had to call technicians from Cubic Transportation Systems, the contractor delivering the Opal card.

But the Cubic technician had told them the issue was a software problem that they did not know how to fix.

On Friday morning at Town Hall station, the gates appeared to be working better than on Wednesday and Thursday.

Even so, station staff were allowing all passengers with paper tickets to walk through the gates unchecked to clear room for a large bank-up of commuters with Opal cards.

For commuters, the problems have been compounded by repeat failures of the Opal website.

People wanting to check the balance on their Opal cards – to see if their cards did tag off properly – have had difficulty doing so because the website crashed a number of times this week.

Transport Minister Gladys Berejiklian, who said last week that Opal glitches were mostly being fixed in a short period of time, said this week that half a million people had now signed up to the Opal card.

Ms Berejiklian has also decided that a range of paper tickets will not be sold from September, including the discount adult off-peak return train ticket.

An organiser at the RTBU, Graham Fozzard, said station staff were having to bear the complaints of commuters frustrated with the Opal card but had not been given responsibility for the card.

The cards are being sold outside the Sydney Trains system; online and over the phone, from private retailers, and from temporary stores at selected stations.

Mr Fozzard said that, at Parramatta station on Thursday, display panels showed that half the gates were not working.

One commuter, Adam Futeran, said he had been overcharged twice this week after tapping off at Central.

Mr Futeran said he called the Opal customer service line - who were polite and refunded his money - but he would not have been refunded had he not noticed the overcharging.

"They told me they are aware of issues at Central Station and are looking into them," Mr Futeran said in an email.

"I then asked would they have picked this up if I had not called and they said no. So apparently it's the traveller’s responsibility to check and advise if there are errors."

A spokesman for Transport for NSW said: "An integral part of any major rollout of this kind is responding to the inevitable minor teething problems that were expected on a project of this scale and complexity.

"Electronic ticketing has been around for decades, so it is completely wrong and misleading to say 'technicians currently do not know how to fix' any issues," the spokesman said.

“Gate Attendant Controller screens located within staff booths at train station gates are used to inform attendants on a range of matters and most of these are not all related to Opal.

“This morning, for example, there were very few issues for customers at Town Hall, Wynyard and Central station gates.”

In a response provided earlier in the week, Transport for NSW said: "Implementing an electronic ticketing system across four modes of transport from Dungog to Bomaderry and Bathurst to Bondi Beach is a large and complex project and the Government has always said that there may be some minor hiccups along the way.

"An integral part of any major rollout of this kind is responding to the inevitable minor teething problems that are expected on a project of this scale and complexity."