There are a number of ways to ensure that when your rainwater runs out, you can switch easily back to municipal water. One method is to fill your tanks with a pre-determined amount of municipal water which remains in the tank at all times. The disadvantage to this method is that it reduces your storage tank size, and you continue to use the pump for all uses, which can use more electricity and is subject to failure when there are power outages.
A switch over from rainwater to municipal is more effective and can be installed for both manual and automatic operation. When your rainwater tank runs empty, your system can be switched over to municipal feed, and completely bypass the tank, ensuring you always have a supply of water even during load shedding.