On climb out of Brisbane the crew had problems with the No 2 engine bleed and at the cruising level of Flight Level 310 were unable to control the cabin pressurization. The aircraft descended rapidly to 10,000ft and returned to Brisbane. The problem was isolated to a failed duct. The aircraft was already operating on an MEL due to the No 1 airconditioning pack being unservicable. Therefore, with the failure of the No 2 pack, pressurization was unable to be maintained. The Company is assessing the need to limit departures with one airconditioning pack inoperative.