There are various reasons why your system is leaking water.
The condensate that is produced in the coils on the indoor part of your system should fall into a drip tray which is either drained away by gravity or pumped outside.
If excess water is finding its way down your wall then it’s more than safe to assume that much or all of the condensate is not being drained away.
Sometimes the drain hole might clog up with dirt, so the water obviously finds the easiest route to drain away which can’t be anything other than down your wall! However, if the condensate is being drained away with help from a pump, then there is every probability that the pump may have failed and this will obviously need replacing quickly. So often we find that the coils in the system have been so caked up with dirt that it has ended up blocking the unit drainage system.
A leak can be caused by something as simple as a dislodged drainage pipe, making the water drain upwards, which, of course, eventually flows back into the unit and down your wall. In this case, it’s a simple matter of correcting the angle of the drainage pipe.
Finally, and in more serious cases, should the system have leaked a large quantity of refrigerant, this will cause the coils on the indoor unit to “ice up” which will also result in leakage when the ice melts. If you do see ice on the coils, it is vital to switch the unit off immediately and call your air conditioning engineer.
Many of these problems can be avoided by regularly maintaining your unit.