If the steamer is overflowing, then there is too much water in the base. Please ensure you keep to the limit of 1.5 litres. Additionally, some types of foods also yield a great deal of liquid during cooking, and when certain types of fish are cooked, they can cause the water to foam.
You can either fill your water tank to the maximum level (approximately 1.5L) for 1h30 minutes usage, or fill it for 20 minutes of use with 0.5L of water. If you choose to use less water, the water will get to temperature quicker, saving you time and making it more environmentally friendly.