@Former-Member 

I would debate that.  It would make every security camera/device illegal.  Obviously you can't go sticking mics and/or camera anywhere to record people without their knowing, but this is where the exceptions would come in.  For example...

1. The parties to a conversation consent to the use of a listening device.
2. A listening device is needed to protect the lawful interests of a person.
3. The use of the device is in the public interest

I would think number 2 here applies to your situation perfectly.  This is from the surveillance fact sheet for the SA Surveillance Devices Act 2016.