I get what you're asking but I think you kind of already know the answer really...
I can't say that I've ever seen it specifically say in any ACT "storage is only in a house" but it's inferred throughout the legislation that "storage" means inside a structure. e.g.
_(b)_ a receptacle that weighs less than 150 kg when empty must be fixed to a wall or floor in a manner that prevents easy removal;
Wall, floor... Obviously the context of the storage requirements is set in a structure.
And frankly it's understood by all that storage means a structure, and a vehicle is for transporting firearms only.
It's not 150kg safe wherever you place it. By that logic you could bolt it to your front porch out in the open or if it weights 150kg dump it on your front lawn.