There are liability issues with volunteers which make it impractical for government to agree to that formally. They essentially have to be contracted or employed in some form. It has been a rent seeking strategy in many instances.
A better strategy would be to properly fund the park ranger service, many of which are indigenous anyway, and commit to a specified level of surveillance and patrol. Rangers have powers to expel people from park areas and know how to liaise with police, but general cost cutting has reduced their effectiveness. Park rangers can also legitimately patrol cultural sites on private land without causing issues with farmers (cattle rustling is still a thing).
A better strategy would be to properly fund the park ranger service, many of which are indigenous anyway, and commit to a specified level of surveillance and patrol. Rangers have powers to expel people from park areas and know how to liaise with police, but general cost cutting has reduced their effectiveness. Park rangers can also legitimately patrol cultural sites on private land without causing issues with farmers (cattle rustling is still a thing).