Sydney's bus drivers say they are encountering an increasing number of gremlins in the Opal Card system, claiming that ticket readers are breaking down more than they ever have.

Transport Minister Gladys Berejiklian has always said that making the public transport smartcard work on buses would be the most difficult element of rolling out the Opal system.

Broken: Bus drivers say the Opal Card system is breaking down more than ever.

But unions representing drivers for both the government-owned State Transit and the private bus networks say Opal readers on buses are malfunctioning at a growing rate.

According to a survey by the Transport Workers Union, which represents drivers for private bus operators, 60 per cent of a sample of 58 drivers said they encountered issues with Opal readers at least once a day.