1) Good dirt jump dirt can typically be obtained free of charge. And dirt jumps should be maintained by riders. This works at public other dirt jump facilities.
2) You have some serious misconceptions about public liability. Council is not legally and financially liable for medical costs. They are not even liable at law (in NSW anyway). Start
here, focus on Division 5.
1) To build jumps you would require at least 3 tipper loads of dirt to make anything worthwhile. Pray tell: Where do you suggest that this amount of dirt and the transport costs involved comes from? It IS NOT FREE.
2) Read your own document:
CIVIL LIABILITY ACT 2002 - SECT 5K
Definitions
5K Definitions
In this Division:
"dangerous recreational activity" means a recreational activity that involves a significant risk of physical harm.
"obvious risk" has the same meaning as it has in Division 4.
"recreational activity" includes:
(a) any sport (whether or not the sport is an organised activity), and
(b) any pursuit or activity engaged in for enjoyment, relaxation or leisure, and
(c) any pursuit or activity engaged in at a place (such as a beach, park or other public open space) where people ordinarily engage in sport or in any pursuit or activity for enjoyment, relaxation or leisure.
This in other words requires that the council or land owner to cover themselves for liability.