It could be a vacuum leak, a split vacuum hose or the like, that's causing the idle mixture to be too lean. It could also be a sensor that's gone on the blink and sending the wrong signals to the computer. You really need to have a mechanic check it for you.

The belt does need to be changed every 60,000 km, but it doesn’t end there, as the belt tensioner, idler and crank sprocket needs to be inspected.

The official fuel economy figure for your car is 8.8 L/100 km, which does suggest that there is something wrong with it. While the official figure is from a dynamometer test done in a laboratory and not necessarily representative of real life, it shouldn't be a long way off the mark as it seems it is in this case. Talk to the dealer and express your concerns to them and ask that they recheck your car and work out a course of action that will identify the cause of the problem, and consider going all the way back to Holden if necessary to get a resolution.