AS/NZS 5601.2:2020 Clause 3.1.8
To prevent risks associated with gas leaks, a “controlled area” surrounds each cylinder. Here cones overlap and control the area above, around and under the cylinder and extend to the ground. The area also wraps around the vehicle.
No ignition source, such as electrical items, or a vehicle opening, or flue, or air intake to any appliance, or any fuel source is allowed within the controlled area.
Equipment essential for vehicle safety is allowed. See Clause 3.1.8 Note 3.

Example of the controlled area around a gas cylinder
The diagram above, as well as Fig. 3.1.8(A) of the standard, shows the cone having an angle of 60° from the horizontal around the base of the cylinder. The 1060mm radius shown at the base of the cylinder is an approximation based on a typical 9kg cylinder. The actual dimension of this radius varies with the height of the cylinder, as a calculation based on the 60 degrees.
So, where does this leave us with a smaller cylinder, for example 4.5kg? Using the calculation based on 60 degrees, the 1060mm dimension will reduce. As each cylinder may have a different height, the calculation will need to be checked for each product type.
The standard allows that equipment required for vehicle safety may be situated within the controlled area. These would include:
No ignition source is allowed in the controlled area. So nothing that could produce sufficient heat, or a spark, is allowed.
Items which are motor driven or contain a switch are considered ignition sources. Such items may include:
Also excluded are appliance air intakes, for any fuel source, and any opening into the vehicle.
AS/NZS 5601.2:2020 allows the use of a vapour barrier as an acceptable technical solution to limit the size of the controlled area. See vapour barriers.
There must also be a warning label, durable and permanent. RV Merchandise catalogue item C-7 ticks all the boxes.

RV Merchandise item C-7, gas information plate
To place an order, use the RV Merchandise order form and catalogue.
For full details check your copy of AS/NZS 5601.2:2020.