There was a busy traffic sequence into Sydney with aircraft being required to hold at Bindook. At 0745 hours the crew of VH-RMD, which was cruising at Flight Level (FL) 390, were told by Sector 7 air traffic control to descend to FL 290 by Bindook. Due to the heavy traffic Arrivals control advised Sector 7 at 0746 hours that Bindook was full and that all arriving aircraft were to be held outside Sydney arrivals control airspace. At 0747 hours the Sector 7 controller told the crew of VH-RMD to descend to FL 350 by 100 miles Sydney, for holding at 100 miles Sydney in a left hand two minute pattern. The aircraft had about 10 miles to run to the 100 mile point and the aircraft systems required re-programing to meet the new clearance limit. There was not enough time allowed by the air traffic control instruction for the crew to be able to meet the requirement. Just one and a half minutes after being advised of the new clearance limit the crew advised air traffic control that they were at 100 miles Sydney and were still at FL 390. The pilot was instructed to turn left onto a heading of 300 degrees and the aircraft was monitored as it descended. No loss of separation occurred.