More than half of the 15,000 breakdowns of Opal readers over the past year have been on buses, leading to a loss of state revenue and confirming Sydney bus drivers' complaints about the smartcard system.

The NSW government's revenue from public transport fell almost 3 per cent in the year to June despite a fare rise and increased patronage, a report from the state's acting auditor-general, Tony Whitfield, shows.

A quarter of all Opal trips – or 74 million – were free, according to the report released on Thursday. About half of all ferry journeys were free, as were about a quarter of trips on buses and trains.

Opal allows commuters unlimited free trips after the first eight paid journeys each week. The free trips last financial year equated to $189 million.