There are very few things as annoying as an auto air conditioning system that is not working properly. In many respects, it would be better if it quit working altogether, as at least then this would force you to take it in to your mechanic. However, you may be dealing with a system that seems to work fine one moment and not the next. What could this be down to?
Problems Caused by Leakage
The first thing to consider is that your system has picked up some unneeded moisture and air and this is causing it to freeze, rather than to continue blowing cold air. Too much moisture will quickly turn to ice and will block the tubing from the condenser. It's possible to get rid of this excess moisture and air by using a vacuum pump. You'll need a special gauge to determine how much air is contained within the system, as once it goes up considerably then there will be a noticeable drop in the performance element and the evaporator could freeze up.
How does air get into the system? Usually, this is through leakage, but it can also be caused when the system is not properly recharged. This calls for the recovery tank fitted to the recycling equipment to be purged from air before being used. When the correct type of equipment is used however, the amount of air in the system will be identified, as well as the presence of other contaminants.
Possible Electrical Issues
It's also possible that electrical issues could be causing your "on and off" cooling situation. The low-pressure cutout switch could be playing up, which could be stopping the compressor from engaging. The compressor clutch could be faulty as well. This means that the compressor won't activate properly and is often caused when the voltage supply is too low.
You might also consider that a temperature sensor is faulty. There are several of these, including one that is linked to the evaporator and one each on the inside and outside of the car that compare interior to ambient conditions. On cars that have automatic systems there could be a problem in the central control module, which would ordinarily throw up a specific fault code to help pinpoint.
Sooner rather than later you're going to have to get the problem identified, which will require a visit to your specialist mechanic. During hot Australian summer days, it's never a good idea to suffer in silence!