The figure is contained in a National Audit Office report about the government's reforms to the probation system, introduced in 2014.
It says high workloads have "reduced" the supervision and training that staff receive and the service they provide.
The Ministry of Justice said the new probation system was "continually improving".
Government changes have seen the probation service split in two, with Community Rehabilitation Companies (CRCs) supervising low and medium-risk offenders and a new National Probation Service (NPS) taking over the supervision of high-risk offenders.
The report said the NPS was "not yet operating as a truly national, sustainable" service and was under-performing in 11 of 18 target levels..
It also found CRCs were not achieving their targets in 12 of 19 measures where "robust" data were available.
Two of the 21 CRCs, which are run by private and voluntary sector organisations, had Â£78,000 deducted from them in total by the Ministry of Justice for failing to meet their targets on the completion of community sentence orders.
The report, which also found "severe" problems with IT systems, said "fundamental issues" needed to be resolved before the new arrangements were capable of achieving value for money.
"The NPS has higher than predicted caseloads and faces a difficult further period of change if it is to play a fully effective role in the transformed and national probation service," the National Audit Office says.
"Its front-line managers face increasing pressure, including dealing with higher than expected workloads, now of high-risk offenders, while assimilating a heavy influx of trainees, who will take time to become fully effective professionals."
A Ministry of Justice spokesman said: "Major transitions in public services are always challenging but figures show the performance of the new probation system, introduced only a year ago, is continually improving.
"As the report notes, the majority of offenders found that services had remained stable or improved since our reforms. Thanks to these reforms, offenders in prison for less than 12 months are now receiving support from the probation service for the very first time.
"However, we are not complacent and are addressing the problems which have been identified. Public protection is our top priority and we will continue to support staff to deliver these important changes."
Some probation staff monitoring offenders in England and Wales are handling more than 70 cases at a time.