In my XG ute the aircon is stuck on the windscreen which is really irritating as I spend all my cold air cooling the hot windscreen and not myself. Does anyone know why this occurs and how I fix it? I have been told that in Holdens if there is a vacuum leak it reverts to the windscreen and it doesn't matter what position the dial is in. I noticed a small water leak in the passenger footwell when the aircon is on so it may be the culprit. Any help is greatly appreciated.

You are right about it being vacuum related. Dont know alot about X-Series, but I imagine the HVAC is similar to E-series.
Check the passenger side of the firewall and look for two hose, one should attach to the heater tap, the other (main vacuum line) should attach to the rear of the intake manifold.

Do the vents, open and close properly, i know when i pulled my dash down there were either 2 or 3 vacum lines in the centre right behind where the controls are, maybe 1 has come off, they were vacumn lines.

I checked the two hoses in the engine bay and there are no splits or leaks around the joins that I can find. I took the air con panel off and all the little plastic hoses seem to be connected behind the dials, and the air con direction dial is clean and ok inside. Any other suggestions?

Yes air to screen is vaccum loss in e and x series. There is also a connector for all lines behind glovebox to be checked. With engine running and controls half out you should have vaccum in from black hose. Any leak may cause a hissing at rotating vac control point. hope this helps!

I've had a look at the lines in the engine bay and behind the dash controls, but not behind the glovebox. I'll check tomorrow but thanks for the info so far.