This suggestion may or may not be in conflict with CASA. My experience comes from road vehicle Australian Design Rule certification.
Firstly, aircraft seat belts wouldn't go 1/3 the way toward meeting vehicle safety requirements. Don't believe me? Look at the size of the bolt holding then to the airframe bracket. Car seatbelts use a 7/16 inch grade 10 bolt (grade 10 is a higher grade than aircraft AN).
This is relevant because it sets the benchmark of how well you need to tie things down. Rule of thumb for a car is 20g.
Secondly, why do you need to tie things down at all? Accident? Things down low on the floor will be restrained by the seats in front. Other things, maybe if you constrain them for a 10g impact then the 5kg bag needs to be held by something that will support 50kg. I reckon 2 strands of kitchen twine will just about do that!
If you want to constrain the items in turbulence, then you only need probably 2g.
Personally, I'd go to supercheap and buy some of the 25mm wide webbing cam lock straps and put them though the seatbelt fitting slots.
Most camping stuff is soft. I'd put the heavy, hard surface stuff on the bottom maybe with some dedicated straps then the soft stuff on top with maybe another strap on top.
If that doesn't do it, you might review if you are really going camping - or moving house
