Welcome aboard.
I don't drink and drive, so I can't help, despite owning an auto D40 STX diesel.
If you're referring to *hic* the diesel-swilling nature of the Navara, a search for 'economy' will turn up a few threads that have nothing to do with Peter Costello and a little bit to do with our cars.
Krafty's absolutely right - first step in achieving economy is to remove your shoes, take the lead out of them and put them back on. There are a couple of technical things you can try:
ECU RESET
This forces the car to relearn the fuel map. In brief: empty the car, take it out to a freeway and stop. Write down your tripmeter if you use that to measure economy, remove the battery NEGATIVE lead, press the BRAKE for 5 seconds, return the battery lead and tighten it then go for a gentle drive - about 10 mins should do - then pull over and turn it all off.
DRIVE GENTLY
Don't use more than 2,500rpm at any time. Always keep the car in 'D'rive with overdrive enabled - this is the only way that the torque converter will lock the drive train to the engine (higher economy).
ENGINE MODS
You might consider an EGR mod as an initial step, and perhaps look at exhaust, DPF, O2 sensor work (there are threads on all of this stuff).
FUEL
Diesel suffers from 'waxing' if it sits too long, so always try to purchase your fuel from a high-turnover outlet like a truck stop. Don't use home-made biodiesel and try to steer clear of bio as much as you can - our engines do prefer dinosaur. Shell, Caltex and BP seem to be the better brands.
Empty on a highway cycle you ought to be able to get somewhere around 10 to 11LPHK. Towing you should expect 15s through to low 20s.
What figures are you getting?