I'm not up with the latest Australian CASA requirements but in the past with S76 ferries we either had a Sikorsky aux tank in the boot or a CASA approved 200 litre drum strapped on a cradle on the back seat which fed into the main tanks by gravity. The extra drums were positioned upright on the floor with the seats removed and packed away. These were hand pumped by a McNaught 1/4 stroke pump into the cradled drum. You simply carried as many drums as weight would allow.
In the 212 the drums were pumped into the 90 gal aux tanks.
The drums were tied down and I don't recall anyone ever having problems although on one occasion the pump was left in the empty drum and the stand pipe vibrated off. Fortunately the engineer had long fingers!
