Just as an idea. If you put the AP on the roof with a PoE device and make your antenna cables as short as possible. The use a second AP with the same setup both with directional entennas you should get the range you want.
Since you are in Auckland let me know if youre interestd in taking a look at my design. I use CHEAP off the shelf gear and have hit ranges of atleast 1.5Km LOS.
Both ends had a "Roll My Own" Antenna one an Omni the other a BiQuad.

andrew