Holden Captiva diesel: Poor fuel consumption

Before buying a new Holden Captiva, Series ll, AWD 2.2-litre diesel we particularly discussed with the salesman our purpose for the vehicle, that it would be used around town, and he recommended the diesel for the best fuel economy. The vehicle is advertised as the most fuel-efficient in its class, with advertised fuel consumption of 8.3 L/100 km combined, 10.6 L/100 km urban, and 7.0 L/100 km extra urban, but we cannot get any better than 12.8 L/100 km. I am very unhappy with the high fuel consumption and have written to Holden and the dealer to have my vehicle tested under supervision, but they continually say it may get getter without actually doing anything to improve the problem. I asked Office of Fair Trading, but they just sent me the Green Vehicle Guide, which again informs me of the fuel consumption my vehicle should be using. I would like a replacement vehicle that will deliver the advertised fuel consumption. Isn't this False Advertising, what are my rights?