Taxpayers are paying "substantial amounts" to Sydney's main private ferry operator for extra services because locals and tourists are rushing to take advantage of a $2.50 cap on fares on Sundays.

Demand for ferries on Sundays to destinations such as Taronga Zoo and stops along the Parramatta River has risen substantially since the $2.50 limit was introduced several years ago. In response, the state's pricing regulator wants to increase ferry fares on Sunday to spread the demand.

Many of Sydney's ferry services now experience their peak loads for the week on Sundays. Credit:James Alcock

Many ferry services now experience their peak loads for the week on Sundays as travellers delay their trips to take advantage of the cheaper fares.

Transport for NSW has told the state's pricing regulator that Harbour City Ferries is receiving "substantial amounts" to provide extra services on Sundays to cope with the demand.