Feral pigs (Sus scrofa) have spread through Namadgi National Park (NNP) in south-eastern Australia since the early 1960s at a mean rate of 4 km/year. Pigs were abundant (approximately 1–2 pigs km−2) during the mid-1980s. Research from 1985 to 2000 inclusive has demonstrated a positive curved relationship across years between the frequency of occurrence of pig rooting and pig abundance (R2=0.48; P<0.001) and the extent of ground rooting; more pigs, more rooting. The ground rooting decreased plant species richness. There was a negative curved relationship between plant species richness and the extent of pig rooting at two sites (R2=0.81; P<0.0001 and R2=0.67; P<0.0001), with plant richness declining to zero with intensive pig rooting. Since the mid 1980s intensive pig control work has resulted in a significant (R2=0.39; P<0.001) decline in pig abundance with an annual instantaneous rate of change (r) of −0.15 between 1985 and 2000 inclusive. The results and their implications for biodiversity conservation and feral pig management are discussed.