Depending on the refrigerant used it is possible that the condenser temperature (the bit exposed to the outside air) exceeded the critical point of the gas at which point it is impossible to tell the difference between liquid or gas. The trouble is phase change cooling works best (most efficient) the closer to the critical point you can go but not past it.
The second problem is the condenser pressure would increase with increasing ambient air temperature. In the past this was enough to stall the compressor motors on a hot day.
My guess is they went for a system with a high efficiency that should work for 99.9% of the time, that last 0.1% is the 8 hours of the year when the temperature is above 42'C (normally for Perth it is normally only an hours before the sea breeze kicks in and drops the temperature by at least 5'C). This time the temperature went up and stayed up for a period of time.