The son of the former prime minister is reported to have set up a business charging 20 per cent a month interest on loans arranged by colleagues who worked "freelance" for him.
There have been claims that the Cape Town business was owed 2.2m rand (pounds 220,000), mostly by soldiers, policemen and low ranking government officials. Mr Thatcher is said to have issued more than 900 loans.
Superintendent John Sterrenberg said: "We are investigating possible breaches of common law and the police code of conduct to do with certain allegations about his actions. We're investigating whether any breach of the law has occurred."
Mr Thatcher was also reported to have made official complaints about some officers not honouring their debts. Some of those who worked for him as agents are facing internal disciplinary proceedings.
The police inquiry, which has been underway for several weeks, came about after Diane Terblanche, chief executive of the Consumer Institute of South Africa, became aware of the operation. She described it as "common loan-sharking".
Mr Thatcher, 45, moved to Cape Town in 1996 from the United States, where he was involved in a number of security-related businesses.
Mr Thatcher is reported to have told one South African newspaper: "People came to me. If I wanted to compete with the opposition, I could do so. But in the event, I was offering a 50 per cent discount from what was available. That's the central issue. Are you blaming me for lending money to people?"
The reports also quoted him as saying: "I'm not the one that elects to increase their debt. They are. What's wrong with that?"
His stately Cape Town home is in the plush Constantia suburb, among the homes of a number of British socialites. He arrived there following a series of business-related problems and costly legal disputes in the United States.Reuse content