Just like any air conditioner, your ducted reverse cycle air conditioning unit can only function effectively if air can flow through it. In fact, one of the more common faults experienced by these units can be caused by something as simple as dust. But how can dust prevent your air conditioner from working effectively?
The wall-mounted ducts of your unit all have filters. These filters (often just behind the duct, depending on the model of your air conditioner) serve two purposes. They prevent the unit from venting dust and other contaminants (such as pollen) into the room. They also stop dust and other debris from entering the ducts via the room. Considering their purpose, it's not much of a surprise that these filters can become dirty. When a filter becomes too dirty, the amount of cold air that can be vented into the room decreases. Cold air then becomes trapped.
When a filter is completely (or even partially) blocked, the cold air intended for the room accumulates directly behind the filter. This chilled air contains water vapour to regulate the relative humidity inside the room. When the temperature of this water vapour sufficiently decreases, ice can form inside the unit's ducts. If a ducted reverse cycle air conditioner doesn't seem to be delivering cold air, closely inspect the filters. There's a good chance that they need cleaning (or replacing).
The ease of removing filters for cleaning depends on the make and model of your unit. It can be easier to just replace the filters, and it's been some years since they were cleaned, replacement is generally a safer bet. If washing the filters, do so in warm, soapy water. Rinse them thoroughly—taking care not to use high-pressure water, which may break the filter's webbing. Allow them to dry before replacing them. And before they're replaced, something else needs to happen.
Once the filters have been removed, any ice behind them will have an opportunity to melt and drain. This will start to happen while you're cleaning or replacing your filters. This drainage should be minimal, but you may want to place buckets or other suitable containers under the ducts while they drain. A few old towels on the floor won't go amiss either, and be sure to move all furniture out of harm's way. Once any dripping has stopped, replace the filters.
If your maintenance efforts don't restore the air conditioner to full functionality, then there's a deeper problem at fault. Please have your unit professionally inspected so the problem can be identified and repaired. But ideally, all you'll need to do is clean your filters, and then make sure they're given ongoing attention to keep them clean.