In a written response to a question posed by an opposition party in parliament, Mthethwa said more than 600 police had been investigated by the service's Independent Complaints Directorate (ICD), which found some had also abused drugs and stolen.
It is unclear how many faced criminal prosecution and if they are still in the police service, said the minister.
"To obtain the information would be time consuming. The redeployment of police employees to carry out this task would be to the detriment of other essential duties or service delivery to the community," said Mthethwa. A copy of his response was obtained by Reuters on Tuesday.
The ICD found 38 policemen "guilty" of murder, 33 of attempted murder and 30 of rape. The highest number of offences was for theft, with 108.
"The total number of members charged (committed crimes) in the financial year 2008/2009 were 669," said Mthethwa.
The findings could raise questions over the credibility of South Africa's police force, charged with tackling one of the world's highest rates of violent crime and making the streets safe before the country hosts the 2010 soccer World Cup. (Please take note Mr. Huntley)
The government hopes the tournament will bring South Africa millions of tourist dollars and international prestige.
There were 18 487 murders, 36 190 rapes, and 14 201 reported carjackings in 2007-8, according to police. About 50 people are murdered each day.
In July, President Jacob Zuma appointed a new tough talking police chief, Bheki Cele, who identified corruption and misconduct among police as a major challenge in fighting crime. (So they admit it themselves)
He has promised to transform an overstretched service tarnished by graft into a well-oiled machine. Cele's predecessor, Jackie Selebi, was suspended in January 2008 and faces a corruption trial in October.
Critics say a lack of personnel and resources have compromised the police service and justice system, allowing many criminals to evade prosecution, with courts often dismissing cases because of poor police investigations.