About 45 minutes after departure, the crew of the Airbus A320 noticed a burning smell in the cockpit and cabin. This was followed by the activation of the `AIR ENG2 BLEED FAULT', `NUMBER 2 PACK FAULT', `LAVATORY SMOKE' and `AVIONICS SMOKE' warnings. At the same time, the Number 1 airconditioning pack began to cycle uncontrollably to an overtemperature of 250 degrees Celsius. The crew commenced an immediate descent to flight level 210 and notified air traffic control. The crew observed that, after selecting the `BLOWER' and `EXTRACT' fans, the avionics smoke warning cleared. The `AIR BLEED 2' fault procedures were then carried out, but the crew was unable to stabilise the No. 1 airconditioning pack. After selecting the `CROSSBLEED' valve to `OFF', the No. 1 airconditioning pack stabilised and the outlet temperature reduced to acceptable values. The flight then proceeded using the No. 1 airconditioning pack. The investigation revealed severe cracking in the air bleed duct assembly aft of the pre-cooler at the Temperature Limitation Thermostat (TLT) sensor-mounting boss for the No. 2 engine. The TLT duct and sensor were replaced. The failure was the subject of a major defect report (MDR) submission to the Civil Aviation Safety Authority.