I'm not sure of the legalities of carrying small lpg bottles inside the vehicle, lots of car based campers obviously do. However the state authorities say to carry them inside a container that is vapour tight to the inside of the vehicle, & vented to the outside of the vehicle. http://www.workcover.nsw.gov.au/form...sheet_0966.pdf

I am more concerned about the bottle being transported on it's side. As I understand it, this is major explosion waiting to happen & an absolute NO NO. The bottle has a pressure relief valve at the top, which MUST be in contact internally with vapour, NOT liquid gas. In the right circumstances (i.e. inside a very hot car) the pressure relief valve can release gas. If the bottle is on it's side, instead of gas being released, liquid gas will be released. A small amount of liquid equals a large amount of gas. One spark, (static, a switch, lighting a smoke etc) & KABOOM!, no more car, driver or passengers. Further I have read that if the valve seal remains in contact with the liquid gas for long periods, that this will eventually cause the seal to leak. I also read somewhere that IF an LPG cylinder has been transported on it's side that it should be stood upright for at least 30 minutes before use.

I am no expert on this stuff, just repeating what those who know have said elsewhere. Better safe than sorry.

Cuppa