I live in central nsw,and have just replaced a natural gas external heating unit with a new more efficient model it will sit under the house where the old one was{for approx 13yrs},My question is the unit will sit approx half meter [500mm] from the gas meter,can I legally use a flex. Hose [rated for external use and professionally made ie;crimped with correct fittings] to supply gas to the unit,this is the only gas appliance in the house.

You need to check with maybe the gas company or someone over management of local utilitys about local codes