I posed this question to a Depi officer on one of my many trips to their offices, his concern was not only people that fall and break a leg in the open hole, but also the wildlife that could drop in there, hard work getting a snake out, kangaroo with a broken leg and so on, he seemed happy enough with the idea of a pellet with a cheapie tarp wrapped around it to cover the hole, so in my haste to dig out the 'welcome nugget' found many moons ago at the depths of Ballarat, I lugged in a chep pellet, brother of dwt came out for a dig, his comment, why didn't you lug in a pine pellet that ways half the amount............DOH!.........
I'm not saying this is 100% legal but a state parks officer was happy enough with my efforts, he did say however that I was not to go below head height, with work safe and other regulations governing hole depths and battering I would of been creating to much land disturbance, at the end of the day, it was deep enough to let me know that there was nothing there 8.(
Ps, State Forest too, not a National Park! 8)