This is normal for evaporative air conditioning units it means your water management or Watermiser system is working correctly. These systems are designed to monitor and control the quality of the water in the tank. So, it only drains water when the system really needs to. It does this by bleeding off water down through the drain, which is replaced with fresh water via water inlet float valve as the water level in the unit drops.
The frequency at which the water manager bleeds off is determined by the quality of the water in the tank. A probe in the tank measures the water salinity (impurities) and, when this rises above a set level, the unit will drain some water.
The frequency at which the water manager allows water to bleed off is dependent on:
1) The water supply quality;
2) The temperature of the day;
3) The humidity of the day;
4) The wind of the day;
5) The fan speed the unit is being run at;
and 6) The size of the unit.