You would be ok providing the construction you built outdoors to accommodate the water heater was built like an interior hot water closet with a roof and a door so it can be accessed for servicing or repairing. You would have to comply also with the ventilation requirements under the Gas Installation Code which are stated in AS5601 and AG601 Section 5.4 and depend on room size, gas input etc. Consideration would also have to be made to ensure there was adequate clearances from combustable surfaces which again are quoted in the Codes. In short the answer is YES subject to compliance with the codes – contact the installer who changed over your water heater recently he will be “Jonny on the spot” and will probably remember your property and what will be needed. Hope this helps.
Selgas Services Ltd Craftsman Gasfitters, Plumbers, Electrical Service Technicians