I'm in the same position: In NZ, trying to build a fixie (although on a road bike).
I went down to a bike shop today and talked to a mechanic for a while. I think if you have a bmx style SS freewheel, it will be screwed onto a 'threaded freewheel hub'. If you unscrew the SS cog, you can screw on a fixed sprocket.
The dodgy loctite method mentioned refers to the use of a strong glue to hold the fixed sprocket in place. It is advisable to put a bottom bracket lock ring on as well to hold it tight. These steps are necessary because otherwise, when you backpedal, the sprocket would fly off (and you would be in the $h^t).
Doing it right: a track hub is specially designed so you dont need these dodgy patch ups to hold the bike together. But they're expensive.
I'm going with the dodgy method because my bank balance is more important than my bodily preservation
and im gonna have a front break.
...just what i've picked up in the last few days, hope i'm not too incorrect...
Check out sheldonbrown.com -it's good
Where in NZ are you? i'm in welly - and ive never seen a fixie either