You fuel economy is off by 30-40% which is very bad. Your air flow meter is not functioning correctly, or your o2 sensor, or your air filter is dirty, or your throttle body and intake manifold area is too caked by carbon, or any combination of all of these. These are the top candidates for poor fuel consumption. Also, check your bellows....this is the air hose that connects your afm to the throttle body. There could be cracks there underneath that you can't notice.
It would be a good idea to get your car scanned by a computer and have any stored error codes read and then deleted. That can zero in on things very very fast. My money is on the O2 sensor and the afm.
Oh btw check your tyre pressures when they are cold (i.e. in the morning, drive to petrol station slowly). You should be running 225kpa front and 240 kpa for the rear. If you have any odd readings, you may have a small puncture - get that sorted out immediately.
Other possibilities for poor fuel consumption would be vacuum leaks (usually detectable by a poor idle when the car is stationary, caused by cracked air hoses), or a dirty fuel system. Change your fuel filter if you haven't done so recently, run fuel injector cleaner through your system (put in 4 litres of diesel on an empty tank and then move to the petrol pump and fill up to full with petrol, the two will mix, no problems for the engine, the diesel will work as a fuel system/injector cleaner). If necessary, you may need to take out your fuel injectors and have them ultrasonically cleaned and backflushed to remove dirt, This is pretty rare, but anyway its a good thing to do at least once every 5 years anyway.
You may have other problems (slight or major) which can only be observed by a trained eye, so tell your mechanic about your poor fuel consumption. Good luck and let us know how it goes. And I would suggest that you get this looked at today, you are wasting tons of money on petrol so every day you don't fix this you're literally losing money.