How this happens is the following:
You get a call from a person claiming to be from the government and they ask you if they can come check your geyser and then if needed to upgrade it free of charge. They then ask you for your latest details / address.
Afterwards, they let you know they will send out their so-called technicians. Now they know your address and then the unsuspecting person opens the door for them and that is when the robbery occurs.
Be very careful. These guys are armed and dangerous and will use whatever means they can to steal from you. Rather get the phone number and call them back and make sure you fully investigate and inform the police as soon as possible.
This is similar to the electricity scam where people ask to check your electricity. In a similar manner, people will phone and say they will give you free energy saving light bulbs and you have to give your old ones to them.
Catching the criminals
Some people actually play along and act all naive. As soon as they are outside the house, simply call the local security company and have them locked up. Do not try take things into your own hands. Rather call the police / local security company to hide around the corner.