The time to treat serrated tussock infestations is always yesterday. If that is not possible, the next best option is to treat the first plant that arrives on your property. There is a single immature serrated tussock plant in the picture, and also a single gorse bush. Imagine the time and money saved if the land manager removed these two plants and monitored the sites for a decade.
Community led action needs to focus on enabling landowners to identify invasive species during the early establishment phase, as well as prevention of spread and pathways. If a rural landowner receives good extension services from community groups/taskforces, these plants would be identified and the risks explained in clear language.