You can get them from Advanced Fuels Technology in Melbourne.
www.advancedfuels.com.au/Its a great way to go from a safety point of view. The specific gravity of CNG at atmospheric pressure is about 0.6 compared to LPG at 1.2, so any leaks will disperse upwards rather than finding a low spot.
New Zealand is very advanced in CNG technology. In Australia, some city buses seem to be the only industry segment that have changed over to using CNG.